Just 2 pins to swap it out and you're done. The only thing easier to field strip is my Baby Eagle. LOL
Ain't quite that simple
I'm going to phrase this really carefully, because my gun nut friends in the US tell me that simply discussing select fire conversions in detail can bring unpleasant attention from the BATF - even if the discussion is only on technical issues and doesn't involve any intention of law breaking. I don't want to get anyone into trouble.
So with that in mind, speaking in general terms, I'm pretty sure of a couple of things.
1) There will be a modification to the receiver of your G3 which would have to be removed before the select fire trigger group would physically fit.
2) The bolt carrier will also have been modified to remove a feature which interacts with a part of the trigger group to make select fire possible. You would need a new bolt carrier, or the original one modifying back to select fire spec. That would have to be done really carefully, because if not then it could potentially allow the weapon to fire before the bolt had locked up, which would probably destroy the rifle and cause serious injury to the shooter.
3) If your barrel doesn't have a fluted chamber then the weapon won't like firing full auto, and you'll likely experience case separations. The G3 delayed blowback design unlocks the bolt while there is still quite a high pressure inside the case compared to other actions. HK's workaround for this was to put fluting in the chamber, sort of like a spline on a driveshaft but in reverse. It equalises the gas pressure by bleeding some back around the outside of the case and reduces the surface area that the brass is sticking to. It'll be really obvious if your chamber is fluted because your spent cases will show the markings. The G3 sniper variants manage fine without a fluted chamber, so it's obviously just a full auto thing.
One thing I might be able to help you with is an optics mount. The old NATO standard was called the STANAG rail and there is a claw mount which attaches to the G3 receiver. They fetch crazy money here ( collectors items ) but it wouldn't be hard to make one that incorporated a Picatinny rail.
I feel kind of bad about not wanting to send you the select fire trigger group, because I'm sure it's possible ( at least under UK export law, don't know about your import regulations ) but I'd have to get involved with government agencies who I am not currently concerned with, and to be quite honest I'm really glad I'm not on their radar. Every year I get a 7 page questionaire from one of them asking about what weapons I import and export and it's great just to be able to say " I don't do that - go bug someone else "
