Intro
I thought i'd put up a page detailing how i went about converting my MC-51 into a G3SBS, but first things first... a little back story. I've been airsofting for a good while, original starting off back in 1993 when i purchased a 8" magnum airsoft model kit for plinking and since then the hobby hasnt really stopped, Admitedly finding the time now is a little bit of a challenge, and of course justifying some of the expense. The MC-51 has been the mainstay of my arsenal since 2001. The main reason being i liked the compactness that the MP5 provided (over the AR) but at the time the only mags i could find were low capacity. When i discoverd there was a cut down G3 that provided the MP5's form factor and the large mag capacity of the G3 (and equal to that of the AK-47) In my mind it was a no brainer, and it always had the unique advantage of being differnent in a world of AK's and ARs. My 51 was been broken, repaired, rebuilt, re-imagined and finally after a long time i decided to take the plunge and convert it into the fabled G3SBS after i found a guy selling the parts i needed to convert it. However it was when they arrived i realised there were a few bits of the conversion i would have done differently so after ordering a new inner handguard, and junking the parts i didn't need I set about modifying my 51 to take the SD front end. So i decided it might be a good idea to put together a small conversion guide after looking at a various different conversions, since i went about a few things differently..
Converting the MC-51 to take the SD front end
What you need
- Tokyo Marui G3 or MC-51 (or one of their clones, Classic Army SAR Offizar, G&G FS-51 etc.) - MP5 SD front end kit (either by chopping up an existing gun, or Classic Army sells a SD front end kit) alternatively if you're buy stuff in parts: - MP5 SD cocking tube - MP5 SD front sight - MP5 SD Handguard holder - MP5 SD Handguard - MP5 SD Handguard clips - MP5 SD Silencer If you can get the Classic Army front end kit, you'll save yourself a world of hassle, since theres a number of screws that are required to hold the front end together, and if you dont have them, it can be a pain to get things to stay together. So far i've found that most parts can be mixed and matched as long as they are parts for a Marui clone, ICS for example have the entire front end availiable as individual parts, so if you were to opt out of the standard handguard and choose a RAS kit like mine (G&G G.A.R.S.) you could save your self a bit of cash. (plus you won't need the MP5 cocking handle that comes with the CA front end kit.)
The first and major part of the conversion process is to modify the MP5 SD's cocking tube to fit the G3 receiver
As you can see in the comparison images between the G3 & MP5 cocking tube, the position and number of mount points is different, the SD foregrip also has an additonal mount point that extends lower than the cocking tube that is used to mount the plastic inner handguard (this is what the handguard attaches to). There's two ways to go about this, one is the quick way that i have seen on the net, and the other is the slightly more precision job which is a good idea if you're going to lug your gun around on the sling, or its largely metal construction like mine.
	The quick way of course of getting the SD cocking to tube to fit the receiver of the G3 is to simple cut down the lugs on the end of the G3 receiver. I've seen a number of guides on the interent where this is what has been done to get the SD cocking tube to fits. Granted it works, but personally i wouldnt be to keen on using it with a sling. Also this reduces the structural strength if you decide you want to go back to using a standard G3/51 front end.
	So in order to retain the two stock mounting bolts, so the cocking tube is firmly locked into place i' ground a groove into the G3 reciever that is just deep enough to allow the handguard mounting points to slot in. By extending the groove further forward slightly it makes it easier to slot the SD cocking tube ito place, by offering it up to the receiver at a 45 degree angle so the cocking tube end can snap into the top of the G3 reciever. While its a bit more work to sit there slowly grinding a groove into the receiver there's a number of advantages to keeping the two stock g3 bolts - if you want to go back to a standard G3/51 frond end its no problem - structural rigidity, which is important if your using a sling, the last thing is you want to have a single point around whicht he front end can flex, becomes more of an issue if you've gone for an all metal front end/body etc. - its somewhat cleaner, so makes selling the g3 body easier if you get bored. Once you've got the cocking tube offered up and fitting nicely, you can you the existing holes on the g3's reciever as guides to drill some mounting holes into the SD cocking tube. with that out of the way... you're half way done (and the hardest part is over)
With the cocking tube out of the way and mounted to the reciever, it is at this point you could swap the cocking handles over, unless you are reusing the exiting G3/MC-51 cocking handle in which case its a fairly painless task of removing if from the old cocking tube, and transferring the handle and inner cocking tube to the new SD cocking tube. Unfortunately i failed to take any pictures of this part. If you're pedantic, it is a good idea to extend the slot in which the cocking handle will reside, since the MP5 being a 9x19 round only moves a short distance compared to the 7.62 rounds  which has a much longer pull, of course this is in real life, and the only purpose the cocking handle serves is to give access to the hop adjustment.  This also saves having to cut any of the cocking handle springs, which makes life easier again if you want to revert to stock. Extending the existing slot by 10mm will give enough lenght to give the apperaance of cocking back for a larger round, but avoids the cocking handle fouling on the front sling loop if the slot was extended the full length of the G3's cocking handle track. Next major part to tackle which will pretty much complete all the modification to be done to get the parts to join is to grind down the inner portion of the SD Handguard holder
	Again like all things there's a quick way and and a slower way... essentially if you want to save time or your sanding/dremel skills aren't up to par you can simply cut off the end section of the SD handguard holder. The problem here though is on the exterior of the rear section of the handguard holder the final 10m are slightly thicker to work as a stop for the outer handguard, this stops it sliding down any further once it has been attached. Additonally it also works as an additional point to add structural rigidity to the handgaurd itself. So, while it might seem an easy way around the problem, its probably best to sand the handgaurd properly so you don't run into any problems at a later date. Properly sanded the full handgauard holder should fit onto the G3 receiver without any problems, and you shouldnt have to modify the G3 receirver in anyway other than the groove for the SD mount points. Plus if you want to go back to a standard front end, the worst thing is to have ugly marks allover the reciever where you've ground it down to slip the handguard over it.
	Now that the inner handguard has had the inner lip, and the MP5 reciever mount point removed, the handguard holder should fit neatly over the front end of the G3 reciever. If it doesnt fit neatly, and bulges out at points you can take note of where there's still material to be removed and gradually sand down any rough sections until the handgaurd is a snug fit. Remember its easier to removal materail than add new material, so sand bits of in stages, and offer up to the reciver until its a good fit. 90% of the time if the fit is a bit of the flexible SD handguard will flex slightly so you wont notice, however the fit is an issue when attempting to fit a metal RAS kit that requires the fit to be close to perfect. As you can see by the picture, i've also transplanted the G3's cocking handle to replace the MP5 handle, for a slightly more authentic look. At some point i'm going to extend the slot that it goes into, so that the g3 cocking handle is pulled a more realisitc distance. For now it does the job.
The Finished Result
Here we go, the end result a shiny new SD version of the trusty MC-51.
Here is my finished HK/MC-51 SD RAS or G3 SBS RAS which ever name takes your fancy.....
Base Gun
-Tokyo Marui MC-51  (to be fair its been rebuilt so many times since 2001, there's not much original Marui Left)   On the plus side its near enough all metal now
Body -G&G G3 Metal Upper Receiver -Classic Army Lower Receiver -Heckler & Koch G3 Drum Site (real steel sight) -Tokyo Marui sliding stock -MP5/G3 Low Scope rail -G&P G3 Grip -G&G Selector switch -Guarder G3 Steel Pins Internals -Laylax Prometheus 6.03 30mm Barrel -Classic Army Metal Hop Up Chamber -Tokyo Marui EG1000 Motor -G&G Gearbox -Guarder Shims -Guarder V2 Cylinder Head -Systema G3 Air Seal Nozzle -ICS Metal Spring Guide -Systema Hop Rubber -Systema 3/4 Teflon Cylinder (MP5) -9.6v Nun-chuck battery	Accessories -G&P L Scope Mount -G&P Aimpoint Killflash -G&P Scope covers -Aimpoint M2 CLone -Aimpoint x3 Magnifier Clone + QD Mount -ICS MC-74 laser -Offset 20mm torch mount -Surefire G2 LED (or custom bored Surefire 6P with CREE XM-L T6 around 800lm!) -Magpul AFG-2 -Many HiCap mags, and/or a C-Mag SD Conversion Parts -Classic Army Cocking Tube -Classic Army Front Site -Classic Army Metal Silencer -G&G FS-51 Metal Cocking handle, and inner cocking tube -ICS Handguard holder -G&G SD G.A.R.S. (RAS KIT) -G&G Rail Covers