GHK GBB BOX AR - AEG gearbox conversion
In the winter of 1998 i was introduced to the world of AEG's and it didn't take long for me to cut my teeth with the purchase of a Tokyo Marui M16, and later on a M4A1. Stock they were fun rifles if a little plastic, and as expected of such a customisable platform, over the years it saw more upgrades than a car park full of Honda's. When GHK released their AR GBB coversion in late 2011, that would allow you to convert a AEG AR-15 into a gas blowback rifle, i was sold right away, as i had been lusting after a more realistic gas blowback rifle for quite some time. So the second they went one sale i had a gearbox ordered not really knowing what i had in store for me! Needless to say this new found love of the AR has led to few more trick parts being added to keep up with the "Jones'" Since info is pretty sparse even a couple of years on, i've collated the experience, and info i've posted up on forums about the GHK kit into one handy page, with info on the G&G version, as well as some info on the beast that is my AR to help navigate the wall of text: -GHK GBB AR Conversion Overview -GHK & G&G GBB Care, Trouble Shooting & Updgrades -GHK vs G&G GBB System Comparison -Spare Part No.s for the GHK GBB Box -My GHK GB AR-15 -AR Gallery Page
GHK GBB AR Conversion Overview
Please bare in mind these pictures were taken a good year and half, after i purchased the GHK kit, i've just placed them back in the box for review purposes! hence the dinged up mag!
Box Contents: Instructions, GHK GBB BOX , GHK AR Magazine, Instructions, Stock nut, Hop-Up Unit, Fire-Selector attachement, Spring for Charging Handle, Grip screw Overview The key peice of the GHK GBB Box, is the gas gearbox itself, it is constucted from a fairly hefty polymer, with the lower section housing the trigger pack, and the upper half the gas system. As system itself its amazingly simple and comprise of only handful of moving parts, the chamber/bolt and bolt carrier. These three pieces are enlcosed in the top housing which is easily removable for maintainence. As such, there's less to go wrong over a traditional AEG gearbox (in some sense,), but more for a headache when you're chasing down exacty which o-ring is causing gas to leak. The magazine's are hefty, weighing around the same weight as a full 5.56 magazine, and like their real steel counterpart hold 30 rounds (40 max, but not recommended!) the downside to this realism is, you do need to care a fair few spare mags which can way you down a bit! The actual construction of the magazines is a steel casing which surrounds a monkey metal core, and two aluminium gas expansion chambers, the idea of which is to reduce the effects off cool down by having the gas already expanded in the secondary chamber so that the pressure is equalised before the next shot is taken (or at least this is my rudimentary observation) They work, and its easy enough to dump a mag in under 2 seconds (while grinnign ear to ear) without the firing rate slowing down on the last few shots. In terms of effeciency if you're not dumping the mag, you should get around 3 mags-ish out of one gassing depending largely on the weather!
Installation
Installation is a very simple process, wether you are fitting a brand new GHK kit, or salvaging a G&G gearbox for your custom build. I've done a breif walk through of the steps to fit the gearbox. I started with the reciever stripped bare so there's no gearbox removing steps (which can very on the make of rifle)
Before fitting the lower half of the gearbox, the selector switch is installed using a special plate that is keyed to fit inot the selector cam. Once this is done, set the selector to "semi" and drop the lower half of the gearbox in. Re-install pins/mag release to hold the gearbox in place.
Attach the grip to the lower reciever, using the provided M6 screw.
The buffer tube is then fitted in the opposite direction from what is normal on AEG, since the gearbox doesn't have the room for a screw to be thread into the back of it. the screw is threaded through the back of the lower reciever, and the buffer tube is held on by a special M5 nut & washer included in the kit. VFC recievers come with a threaded section on the back of the reciever, so these can be a bit of a pain to screw a bolt in the wrong way (since you're screwing through two sets of threads). I ended up using a metal plate and a standard hex nut that could be tightened from the rear end of the buffer tube in the traditional way, since the screw had already been tightend as much as possible in the reverse direction
	Next, the remove the standard aeg spring, and plastic lever (used for opening the dust cover) from the charging handle you are planning to use and fit the included spring to it as in the photo. -the included GHK spring, does deform very easily, I over stretched mine the first few times i cocked the rifle. The springs can be replaced by modifying the stanard charging handle spring to fit over the peg on the top half of the reciever. -charging handles can fail to catch with the tab on the top of the bolt carrier, so it is best to find one that has a decent sized lip at the end, so it can engage the bolt carrier properly. -depending on the reciever and/or charing handle there can be a fair bit of wobble. I found SRC and VFC charging handles to have a nice tight fit, unlike Guarder and G&G which felt a little bit more loose. If there was ever a reason to get a Crusader BCM charging handle it's this. -DO not try and for the assembly in with the dust cover flap lever..... I've done it, and it meant the whole lot got stuck in the top receiver and took and age to take out., you can snap the lever piece down, so it locsk the spring onto the charging handle. Personaly i think it would have been nicer if a real steel style (length) charging handle could have been used, as this would have elimated some of the wobble at full extension.
Here is the hop-up unit assembled, and as it comes, it is a farily simple affair, comprising of a rotary adjustment kob that holds the hop setting pretty well, and as of yet i've not had any problems with it loosing its setting despite the recoil.
Initially its a bit of a pain to assemble since the holes the steel pin is meant to be inserted into, so that the arm can pivot is a bit too small (not a bad thing mind, as it won't fall out easily). The unit itself is signifciantly shorter than what is typically used on a AEG, mainly because of the RS sized magazines, fitting is the same as regular hop up unit and will fit most if not all AEG outer barrels without any hassle. Being short is an advantage when you're maintaing the gearbox since its easier to check if you're getting a decent airseal in the hop since the nozzle should seat nicely inside the hop.
With the Hop-Up unit installed, all that is required is that the top portion of the gearbox is slid into position into the upper reciever, the the two receiver halves are ready to be mated. All that needs done is re-installing any furtniture or accesories removed before the conversion, a magazine filled with gas and the conversion is done.
Fully Assembled	The simple system allows for quick change uppers to a suit your mood.
Performance & Reliablility Once re-assembled, and with a full mag i found the standard GHK was firing at around 350-330 fps (this was in around 19-20 degrees). I've gone further into the performance in the comparison between the G&G and GHk gearboxes. Trigger response is crips as would be expected from a mechanical system that does not have to pre-cock the spring, with a standard rate of fire of around 16 rps (19rps after a new spring was installed). Each pull of the trigger is followed by a decent crack as the gas leaves the barrel, as well as the metallic noises of the bolt returning to battery etc. A huge improvement over the sound of a V2 gearbox whirring away. Recoil is good, since this depends largely on the weight of the rifle the gearbox is going into, its high subjective, but i found with a farily un-gucci 12" M4 the recoil to be almost equal to firing a .22lr, in full auto the gun does jostle about, but nowhere to the extents of a full fat GBBR. Overall it is not a bad replacement for your average V2 gearbox, and can keep up with other site legal rifles, while being a lot more itimidating due to the sound. So far in the past year and half, i've put 20-25k .25bbs through the gearbox and althought i've had a few minor issues with consumables like o-rings (worn or unseated), and minor mag leaks. nothing so far has caused any major problems that have left the rifle unusable.
Conclusion Out of the box it brings a new lease of life to the AEG AR platform (at least for me), especially for those who would like to move on to more realistic rifles (both in fuction and size) but don't want to outlay a lot of money for the horror stories that GBBR's can bring, or just want to have a more fun rifle that gives a bit more user feedback than a high pitched whine. I'll end it with some of the pro's and cons of the system, ultimately i found it a good piece of kit, and after puttting alot of BBs down range with it, i'm quite happy, even if the call of a RA-Tech M4 still echos in my ears. Advantages -AEG based, so bar the gearbox itself, there is a wealth of aftermarket parts that can be fitted without any problems, and works your existing AEG -Spares are availble from sites such as WGCShop, and its easy to build up quick change uppers to suit your game play -Recoil is fairly solid for such a simple system, you won't be acheiving WE/WA levels of recoil but its enough to be felt and give positive feedback when firing -Fast Trigger Response due to its mechanical nature -Decent cyclic rate without having to worry about gears, motor -Replaces the V2 gearbox as a unit, so a user can switch betwen AEG and GBB fairly quickly if they know their way around a rifle. -Realistic mag capacity, though for some this might be a disadvantage, but it does make you think about your shots. -Simple system, there are very few moving parts to go wrong, as long as its maintained properly it should last a signficant amount of time -Batteries not require, no more trying to find a place to hide a decent capacity battery -Noise! No more "zip, zip" noises as you pull the trigger, the gun fires with a satisfying crack, increased with a shorter barrrel and a sound amplifier (like a Noveske) Disadvantages -being a GBB, temperature and cooldown are factors in the rifles performance, and in extreme cold/heat you'll end up with a rifle that doesn't work or fires too hot  saying that, the GHK system does handle cool temperatures quite well especially with the standard GHK nozzle. -Unstable FPS, unlike a AEG which displaces a set volume of air, -AEG based, this has its disadvatages from full GBBRs because the recievers are not 1:1 scale so there are many RS parts that do not fit like grips, selectors etc. -Low Recoil compared to full GBBRs -Non functioning bolt catch, improved on G&G V2 gearboxes -Non functioning dust cover, G&G has fixed this -Bolt carrier only travels 20mm, rather than the full distance, when firing this isnt so much of an issue, but on firing the last roung the bolt carrier only stays locked back about 10mm -Temperamental if poorly maintained, you'll need to make sure you properly clean and lube the gun, especially if you're going to leave it stored away at long periods of a time, the o-rings etc an crack, and the mags can leak if not gassed. -leaky mags, overall the GHK magazine design is pretty good, but they can leak from either the two sides of the main valve/filler or from the bases of the two cylinders of the gas chamber, their are all easy fixes..... but can be a problem if it occurs at the wrong time -Mag followers are pretty fragile, as is the tab that keeps the BB in the mag. Probably not best to practice you're operator skills when it comes to mag changes, because one false move can easily snap off a piece of the mag, and you're left out of pocket (but it does mean you have spares, for when you're mags leak
If you don't do anything else, run a short barrel and a Noveske KFH or KX3 and revel in the glorious noise as you pull the trigger!
GHK & G&G GBB Care, Trouble Shooting & Upgrades
DIY upgrades & Maintainence
Lubrication -Silicon lube and keep the o-rings happy. This is probably the most imporant part of caring for you gun, and getting the best performance out of it. While too much silicone lube can cause the o-rings to swell, too little and the gun will fail to cycle, or even fire. -One way of telling if you gun is in need of some lubrication is it will sound different when fired, instead of a loud report on firing, you will get a quiter tinny metallic sound as the gun is fired. -Since the bolt carrier slides around inside the upper reciever it is best to make sure there are no parts that can snag, and that its outer surface is lubed to stop frcition between the carrier and the reciever body. On the original metal body i used i found the G&G bolt carrier would catch the reciever due to its pronounced lip for opening the dust cover, and i ended up sanding it flat to reduce friction.
Hop-UP -The hop up will accept standard AEG hop up buckings, so getting a decent hop rubber/nub has its benefits. -PTFE Tape, the mainstay of airsoft, using this to help create the best possible airseal around the hop up will help keep the FPS consistant. -The rear section of the hop-up unit has a retaining o-ring for the hop selector, I've found that sandwiching another o-ring between the top section of the gearbox and the hop rubber, helps to stop air leaks which helps maintain FPS and recoil.
Barrel -Unlike other GBBR's on the market since the GHK system is designed as a drop-in replacement for an AEG gearbox, AEG inner barrels are a direct fit. One of the first mods to do that will increase FPS & accuracy is get a good quality tightbore barrel from a company like Prometheus. Generally with Gas guns, due to the difference in gas expansion from a AEG pison,  a longer barrel will increase FPS, if only slightly. I have found that my 460mm inner barrel shoots around 10fps faster than my 300mm barrel (but other factors like temperature/cool down etc.  could have an effect on this finding)
O-rings -The o-rings on the bottom of the chamber should probably be relplaced with thicker/tighter ones so they don't shift when the mag is inserted. Its quite easy to knock this o-ring off during an enthusiastic mag change resulting the in the gun venting the mag when its fired.
-Bolt/chamber o-rings, these can be replaced for a slightly better seal without causing moving parts to get stuck. Stock the air seal isnt too bad but there's always room for improvement. You do need to make sure that the bolt can move freely withing the chamber or else you will end up with cycling problems!
Ghetto ROF increase If you're too cheap to buy a high velocity spring, you can insert a 5p coing in the back of the upper portion of the casing betwen the spacing and the back face. This will increase the compresion of the spring and force it to cycle faster. It does have its flaws though, too many coins will cause a jam as there will not be enough space for the bolt carrier to travel backwards.
Trouble Shooting
Venting WTF?? THis can happen for a number of reasons. -low gas in mag = not enough pressure to cycle bolt so gas release arm will remain in the forward position -low ambient/mag temperature, same as low gas in mag -o-ring damage to the o-ring on the base of the chamber where it meets the mag from over enthusiastic mag changes (or over time from mag changes) -bad nozzle seal (see below) -play in gearbox halves (see below) -under lubrication, noticable since the bolt etc will take more effort to move, (make sure the trigger pack is lubed so the arm doesn't stick, and the bolt moves freely) -over lubrication after a long day of play using gas with lubricants, this can gunk up the chamber causing the large o-ring between the chamber and bolt carrier to start slipping from its position causing an inadequate air seal.
Leaking Mags Mags can and do leak unfortunately, but on a positive note due to the GHK design there are only 3 places to leak from an repairs are pretty simple. -Fill and knocker valves can be stripped and their o-rings soaked in silicone oil like any standard GBB valve. Replacement knocker and feed valve, bother parts are availiable seperately if you don't want to attempt to repair them. The knocker valve is the same as used on the GHK AK magaziines, and are availaible from Samoon / WGCShop, and the fill valves are standard pistol valves. -Gas Chamber, this can leak through the bottom of the mag and a simple fix. Remove the base plate of the magazine, and the two allen screws on each side of the magazine near the top. The casing should then be removable by sliding it downwards from the gas cylinders. Remove the caps on each gas cylinder using a 6mm hex key and re-seal with either new o-rings/ptfe tape or sealant. An extra bit of PTFE tape can be wrapped around the bases of the cylinders once the cylinder has been re-assembled.
Full Auto Not Working Similar cause to gneral venting problems so some of this is a copy and paste job -low gas in mag = not enough pressure to cycle bolt so gas release arm will remain in the forward position -low ambient/mag temperature, same as low gas in mag -o-ring damage to the o-ring on the base of the chamber where it meets the mag from over enthusiastic mag changes (or over time from mag changes) -under lubrication, noticable since the bolt etc will take more effort to move, (make sure the trigger pack is lubed so the arm doesn't stick, and the bolt moves freely) -over lubrication after a long day of play using gas with lubricants, this can gunk up the chamber causing the large o-ring between the chamber and bolt carrier to start slipping from its position causing an inadequate air seal. Seems crazy but i have encoutered this quite a few times, after prolongued usage. Alternate betweeen a non lubricated gas like Abbey sniper / pure proplane with regular lubed gas mags. (see the section below on bolt removal, for a guide on how to remove the reciever and clean any excess lube off of it) -Has the bolt spring been cut down? this can cause issues with the rifle cycling. While cutting down this spring can help in COLD weather to reduce the amount of gas require to cycle the bolt carrier, and/or for increasing felt recoil, this can can have a negative effect where it cause the bolt carrier to take longer to return to battery causing the gas release arm to remain in its forward position. -Bolt Carrier rubbing against upper reciever, Depending on the reciever and tolerances this can be an issue. Test the bolt carrier has a smooth action when cocked/back and fort, apply silicone oil to the outside of the bolt carrier to ensure it moves smoothly within the reciever, additionaly see below about the additon of spacer between the top half ot the gearbox and upper reciever.
	Cannot Cock rifle / Trigger Jam This can happen if there is too much play in the upper and lower reicever, or if the gearbox has been re-installed incorrectly. -make sure the full auto sear (circled) is pushed into its forward position with the trigger sear pushed down into its "cocked" position.
	Nozzle Air Seal -The "bolt carrier" can be sanded slightly where it hits the body if the nozzle isn't getting inserted decently into the hop, best way to check is pull the trigger so the nozzle is in the most forward position and look into the magwell, if you can see the brass lip of the nozzle you're probably not getting the best seal possible So carefully sand the front of the "bolt carrier" by a hair until the nozzle is meshes with the hop rubber and creates a better air seal. This is probably not necessary on a stock G&G gun, as that's somethign they would have checked for when designing their GBBR. However if you are using the GHK/G&G box with an aftermarket metal reciever you may run into problems in this area.
	Bolt Carrier Spacer Particularily on guns with a custom reciever the bolt carrier may foul agaist the upper reciever, causing it to slow down, or fail to cycle. This is because AEG recievers are generally not designed to have bits flying around inside of them. The way i got round this was to place a 1-2mm sliver of plastic inbetween the bottom of the top portion of the gearbox and the lower edge of the gearbox (near where the rear body pin is located). This forces the gearbox ever so slightly to the left which gives better clearance for the bolt carrier as it moves backwards. This may just be down to the tolerances of the reciever i use, as other recievers i've tested have less problems in this respect.
	Unable to fire, Gas Venting from Mag on Trigger Pull Unlikely scenario on a G&G Combat machine, but when using an aftermarket body where the tolerances for the recievers are different this could be an issue. The gun may fail to fire due to the upper and lower portions of the reiver not mating together properly, this can cause problems with cycling where the sears are not getting engaged as the "bolt carrier" is cycled, or venting of gas as the nozzle will not be fully inserted into the hop up. To check if this is the problem, with the rifle assembled test if the upper portion of the gear box has noticable play via the ejection port. If it does you can add a spacer to the top of the charging handle post as recommended by GHK. However in the case of the Hurricane HKM4D body i had to sand down the raised portion where the charging handle screw mounts to as it was pushing the gearbox too far down, making it a tight squeeze on assembly and causing a feed issue where the nozzle would not correctly enter the hop up chamber.
	Sloppy Charging Handle This can be a bit annoying, and it can lead to the charging handle not cocking the rifle and/or damage to the charging handle spring if it gets over extended when cocking the rifle. (replacement springs, i've mentioned above) Two things can causes this, a gap between the upper reciever and charing handle when it is installed, and not enough of a lip in the central channel of the chargning handle. In order to combat this, a peice of perspex or similar glued to the top of the upper reciever will stop the chargning handle moving about to much vertically. I've tried a few charging handles, and the ones that have the best tolerances out of the box are SRC, VFC and Crusader charging handles. I found that Guarder, G&G and Tokyo Marui charging handles are generally not as "tall" so tend to be a bit sloppy and chan lift off the nub on the top of the upper reicever causing a failure to cock (probably down to the fact that AEG chargning handles are just for show!)
Aftermarket Upgrade Parts
FPS Upgrades The bolt / nozzle can be upgraded/downgraded depending on if you have a GHK or G&G gearbox. I've included a rough guide of the FPS you can expect with each of the different bolts, but the actually FPS you may vary (especially in the hotter weather!) The higher fps bolt/nozzles do have their uses in a full auto gun, particularily in the winter where the standard bolt will be unable to keep up with the FPS of the AEG's It goes without saying, its advisable to test the fps of your rifle in different conditions so you don't get caught out with a gun that shoots to hot for your site.
Standard GHK Nozzle (comes with GHK BOX or availiable as a spare part)	fresh mag, 380fps, levels out to around 330fps
Standard G&G Nozzle (comes as standard with G&G GBBR)	fresh mag 440fps, levlels out to around 380fps
GHK High Velocity Bolt (availiable seperately)	fresh mag approx 520fps, levels off to around 450fps
I've put together some illustrations & guide below of the "bolt"/nozzle upgrade procedure
	Re-assembly Once you have seperated the chamber from the top portion of the gerabox, its a fairly simple process to remove the "bolt" nozzle from the chamber. When re-installing the nozzlethere is a spring inside the rear portion with loops that hook onto the retaining pin at the back of the chamber. In order to re-install this, take a paperclip or any thing gauge wire, and bend it into a hook shape at the end so you can use it to catch the loop and pull it into position when re-inserting the retaining pin at the end of the chamber. In its rest position the "bolt"/nozzle should be at its furthest poing back in the chamber, and when you push it forward it will spring back when released. In practice firing takes place when the nozzle is fully extended into the hop unit, and retracts for the next BB to chamber. It should be a smooth action when you push the nozzle in and out of the chamber or there is a chance there will be feeding problems. While the upper portion is dissambled it is a good oportunity to lube any of the o-rings, and replace any that are worn. One thing to bare in mind is that the bolt bolt carrier are both moving parts so make sure they move with minimal resistance while maintaining a good airseal. Putting in too large an o-ring can cause too much friction which will in turn slow down ROF or the gearbox will fail to cycle. The rest of the process is reversed from then on, and should only take a few minutes to put back together.
Higher" ROF spring -This is the main spring that propells the bolt carrier back after it has been sent backwards by the gas, similar in principle to spring in the buffer spring in a real steel AR which helps cycle the bolt back into battery. Can be bought seperately or with the high veloctity bolt (mentioned above). The stock cyclic rate is around 16rps, but the spring can increase it to around 19rps. However the rate of fire is dependant on gas pressure, so leaks or low temperature can have drastic effects to the rate of fire.
	Selector Cam -This part can be bought seperately, for conversion to semi auto, or alternatively the central spur can chopped to down to single shot for sniper/dmr use, particularily when using the high FPS bolt. (probably best to do this to a spare part, as its a permament modification.
	High Flow magazine valves -I've ordered the Action High flow valves for GHK AK magazines, and they fit perfectly. Having taken them apart the only difference is the larger ports on the valve, and the way the circlips hold the valve together. The springs "feel" to have about the same stiffness as the stock valves. Having tested them, and it might be a due to a flaw in my testing but i found not discernable difference in FPS or cycle speed betweent the stock GHK valves and the Action valves. But they are a good replacement if you damage a valve and need to get a spare.
Untested Upgrades
Weighting the "bolt carrier" for more recoil Now that i've finally got a spare one, i've yet to test the theory that it would be possible to replace the cylindrical portion at the rear with steel bar to increase the weight slightly). Potentially the whole part could be machined out of Steel rather than the pot metal that the stock boltcarrier is made from as an alternative root to adding weight. The downside is... The gun will use more gas, and reduce ROF since the increase of mass will reduce the cycling speed of the bolt, but the results could be entertaining!
Reprofiling the valve part of the bolt, This seems to have the biggest effect on fps as these are the only differences between the GHK and G&G gearboxes, in theory you could be able to bump up the FPS by directing more gas towards the BB... but that could have a knock on effect on the ability to cycle the gearbox.
GHK vs G&G GBB System Comparsion
	I've been curious to see the changes that G&G have done, ever since they announced they were manufacturing a GBB version of the Comabat Machine using the GHK AR BOX as a base. They reported they had improved on the original GHK system and that it wouldnt be identical, so when i had a chance to get hold of a G&G gearbox it was a great opportunity to do a comparison between the two systems, and it gave me a second gearbox so i could have a spare upper half with a different "bolt" so i could up/dowgrade fps with minimal fuss. At first glance the two gearboxes (to help the GHK box is the one with the semi-polished bolt carrier), the molding and material of the shell is identical. The only major differnce to outer shell is that the top half of the G&G geabox has additional screws added to hold the bolt carrier, a negligible improvement but probably strengthens the top sectioin in the long run
The first major difference to the two gearboxes is the inclusion of a lip at the end of the bolt carrier, so that it can catch the dust cover so it opens when the rifle is cocked, one of annoying part of the original GHK design as it meant the dust cover had to be permanently open, and if you closed it by accident you had to strip the rifle down to re-open it. The second difference is the G&G bolt carrier has two drilled holes in the bottom, GHK doesnt. Unfortunately i'm not sure if the spring in the G&G box had been modified by its previous owner, so i couldn't really compare that part (length/strength) as it looked like it had a coil or two trimmed off, which in turn caused cycling problems when i initially tested it, although did increase the recoil. This was replaced with the spring that came with GHK "high veloctity" set. Cycling on the GHK is around 16rps, and 19rps with the high veloctiy spring), which isnt bad at all, just wish i could compare it to a stock G&G.
Strangely though, despite their differences, both bolt carriers weigh more or less the same, so the additional material on the end of the G&G is offset by the material drilled out  What this means is, with the same spring & bolt the recoil will be identical between the two. Some people have reported that the recoil feels stronger on the G&G, but this is probably down to the polymer recivers that the Combat Machine's have and overal lighter weight of G&G rifles, when the two were tested in my gun the recoil felt the same between the two.
	Chamber on both is identical (not sure what this part is called)  both do come with pretty shady o-rings on the part that connects with the mag, which are prone to slipping off if you're to energetic with you mag change (and results in the mag just venting... or failing to fire in anything other than a weak fps semi). I noticed above someone had mentioned the gun just venting after chaning mag, and i think this might have been the cause.  I've changed them both to a slightly thicker o-ring in the photo, (normally the G&G one is green)
	The biggest difference between the two geaboxes is the actual bolt itself, compared to the stanard GHK bolt the valve part of the G&G bolt has been re-profiled to allow alot more gas to divert towards the bb, if you look at the picture you can see the G&G has a far steeper profile than the two GHK bolts (the GHK high veloctity bolt uses a spring at the front part to reduce the restriction on the gas flow) This part is probably not that scientific, or accurate.... but here are my findings when chrono'd, done indoors at around 20 degrees, freshly gassed mag each time i changed the bolt, tried to keep the time between shots constant, so cooldown was equal between tests) 433mm 6.03 tightbore, ghk hop, madbull shark rubber and only the bolt was changed to test), abbey predator gas. -Stock GHK - fresh mag, 380fps, levels out to around 330fps -Stock G&G - fresh mag 440fps, levlels out to around 380fps -GHK high velocticy - fresh mag approx 520fps, levels off to around 450fps Luckily you can get both GHK bolts seperately so if you need to reduce your fps for CQB use its do-able, or increase it for DMR usage One thing i found in during the particularily hot summer, is that the G&G for a stock gun shoots ridiculoulsy hot in elevated temperatures, and was average around 420fps+ in 25-28 degree weather. So if you're looking for ways to reduce the fps of your stock G&G, then the best bet is too order a GHK standard bolt (part no. GHK-PT-M4BOX-05)
As for the rest... Trigger pack Identical, fortunately GHK make a semi-auto only selector if you plan on using the "high velocity bolt", didn't strip this down as there's not much to look at here. Magazines Identical, from the spot welds to the markings on the bolts. I have a feeling these probably come direct from GHK, and G&G just repackage them.
**UPDATE** Since writing this, G&G have brought out a version two of their GBB system, which allows for a fuctioning bolt release catch, the "bolt carrier" is slightly re-profiles at the bottom leading edge so it can be caughty by the bolt release catch when the mag is empty, the V2 magazines as well have been changed to incorporate a lever that allows the bolt catch to function. The only other change has been to the selector, so its is now ambidextrous.
Spare part numbers for the GHK GBB Box
Hop-Up Chamber (useful for multiple barrel/uppers)	GHK-PT-M4BOX-04
GHK Loading Nozzle/Bolt (standard fps)	GHK-PT-M4BOX-05
GHK Loading Chamber	GHK-PT-M4BOX-06
Spare Magazing Valves	Same as valves for the GHK AK-47 Magazines
MY GHK GBB AR-15
The rough spec-list follows, not to say by now it hasnt changed (even while writing this, i've gone a stuck a 11.5"barrel onto the 16" SPR upper, so i could tuck a Noveske underneath the rail for a bit of fun)
Base -Tokyo Marui M16A2 AEG Internals -GHK GBB AR Box   -Polished GHK Bolt Carrier + GHK Standard Bolt (320-360fps)   -Polished G&G Bolt Carrier + G&G Bolt (360-420fps)   -GHK High Velocity bolt (450fps+) Lower Reciever -Avalon/VFC Bravo Company BCM-4 Lower Reciever -Crusader B.A.D.A.S.S. selector -Crusader Anti-rotational Links -Mil-Spec Buffer Tube -Norgon Ambi mag catch -Magpul MOE grip (FDE) -KAC Combat Trigger Guard* -Vltor Emod Stock* -Magpul B.A.D. Lever* -Vltor SASE-2e Sling Adapter* Upper Reciever -Avalon/VFC Bravo Company BCM-4 Upper Reciever +Madbull 12.5" Daniel Defense Govt. Profile Steel Outer Barrel   -318mm Lylax Prometheus mm stainless steel barrel   -Madbull Shark hop-up bucking   -G&P Vltor Gas Block   -Systema A2 Front Site -Crusader BCM/Vltor Mod.3 Charging Handle -Celcius CTW Delta Ring / Barrel Nut -PTS Battlecomp 2.0 -Vltor CASV-MT* -A.R.M.S. #40L Flip-up BUIS* -TangoDown BGV-MK46K Stubby Vertical Grip (FDE)*	Optics -Eotech 552.A65 Revision F.*   -GG&G Flip covers*   -GG&G 551/552 Accucam* Misc Accessories -Surefire M952v Weaponlight* -Haley Strategic Mil620 Mount* -Magpul 5.56 Magpuls (FDE)* -Ergo/Magpul Ladder Rail Covers (FDE)* *Genuine Real Steel Items