Re: HK91, have a big box of parts
The diagram above posted by E.P. should be the right one.
It looks like the bolt carrier++ and a handful of small parts are missing, but it should be relatively easy to come by those over there, as several companies makes aftermarket stuff that "should" fit.
My experience with those babies is limited to the original H&K with 3 settings on the safety knob.
To compare it to the M16 an its variants, and the current H&K416 that almost completely have replaced it, is kinda apples to oranges as the scenarios have changed over the years since it was implemented as the main combat rifle over here, back then (late 60s,early 70s) the choice of caliber and rifle made very good sense, and that is still one of my top favorites.
The punch, reliability and accuracy it packs, that has made it "famous" and feared not least in the middle east over the years, is excellent.
The only major drawback imho is the lack of a built-in Picatinny-rail (although a genuine H&K claw mount is never wrong) but the stock iron sights (with the rear "drum") would allow for very good precision to at least 300+ yds shooting controlled SA.
Its relatively light and that means FA is kinda interesting, shooting strings of more than 3-5 rounds would be ineffective as the rest would most likely go high, in situations other than very controlled FA at the range... compared to the M4 or 416, that are extremely manageable even when dumping whole mags downrange, the G3 does not call for that kind of ammo consumption
Today it would,equipped with current optics and other gadgets, fit the role as an SDM rifle perfectly,imho.
Just a little nostalgic ranting
Good luck on your build, hope it works out for you and you shoot some nice pics for us, thats one heck of a rifle once its set up.
John - out