My good friend CJ is a real whiz at building H&K roller locked stuff. Back in the day when G3 parts kits were affordable and flowed into the United States like water anyone could build one for a few hundred bucks. The problem however (aside from relative build difficulty) was that a true G3 spec chamber does not like commercial .308 Winchester ammunition. G3 rifles have an internally fluted barrel. This aides in extraction and prevents the possibility of an extractor breaking, and a shell sticking. The gas from the discharge is forced back and around the spent shell blowing it out of the breech. So, in the event an extractor breaks, or some ammunition failure, you will definitely get the spent case out of the weapon. With no extractor at all it will eject a round, or at worse cause a partial ejection which requires you to pull the cocking handle and drop the round out. The problem with this is it mangles brass. CJ found this out the hard way:
What you see here is what can happen. This was commercial .308 fired in a properly built G3; It is very dangerous and can kill you. You must use 7.62×51 NATO if you have a true G3 spec barrel. As you can see, the top of that round has the aforementioned flute marks – that’s what a spent shell looks like – it gets ruined. Now, if you notice… that’s only 1/3 of a shell, the rest of it was ripped off. Since commercial .308 is much thinner cased, when the round discharges the more pliable case expands and actually sticks in the flutes, and given the power of the round, the extractor literally rips the back of the case off, leaving the front of the shell stuck in the barrel. The next round was cycled in with such force, it actually slid itself through the remaining case neck. Luckily CJ noticed the rifle wasn’t in full battery and didn’t fire it (that would have been catastrophic).
H&K 91 rifles had a much looser chamber to allow the firing of .308 ammunition, and modern manufacturers of G3 pattern rifles seem to have adopted a similar chamber. Just beware of what can happen and be safe!
Special thanks to CJ for sharing this information and photograph.