i am going to tell you why the ar-15 system is so influential, even though aspects of it are not universally revered or even particularly admired.
but, first a digression.--
go to this lovely website, run by a crazy russian whose english language syntax makes boris badenov sound like sir john gielgud, here, at http://world.guns.ru/assault-e.html . the website host may fracture english, but he writes "guns" just fine, and in a very sprightly and opinionated fashion.
he gets his points across.
and, go to those portions of his website that deal with the transition of the main battle rifle from the bolt action mauser dominated world, to the beginning semi automatic rifles such as the germans and russians and americans made.
the early efforts were brutally complicated systems, and the fire control systems were the things of genius, ... , but, they didn't work so very reliably, simply put. there were all sorts of brilliantly contrived mechanical linkages and this and that sort of contrivance designed to work with the bolt and breaching system to control the manner of fire, and whether it was done from an open bolt or a closed bolt, ... , and, yada yada yada.
it is worth spending a copy of evenings just looking at the efforts of some of the world's brightest people, and the abysmal failures issued forth by their efforts.
now, let's come back to the ar-15.
with the ar-15, the genius is found in the trigger. and, the fact that the bolt and the bolt carrier have absolutely nothing to do, basically, with fire control. as a result, the ar-15 is the most ridiculously flexible system ever designed. maybe it is not the best at everything, but, with a little tinkering, it can be made to do just about anything.
for instance, i am a devoted fan of the russian rpd light machine gun, designed by the russian degtyarov. http://world.guns.ru/machine/rus/degtyarov-rpd-e.html . it is a marvelously clever and robust system, and a dedicated machine gun that fires only from the "open bolt" position. i think it so mechanically fascinating, that i considered buying a kit and a receiver and obtaining a federal license to build one, and would have simply for the fun of building one, in all its glorious complexity. (yeah, when pigs fly and i could scrape up enough money in the next five years. i may have a hamburger this week, if i can afford that. laughing.)
the rpd is one of the best purpose built light machine guns, ever, by anybody. period.
and, then i got to thinking.
i can duplicate it with an ar-15 build, if i want a squad light machine gun all that badly. you know, for fighting off the chinese hordes with about five lifetimes of ammunition to squander. (to that end, i was reading at the mad russian's place about the bren light/intermediate machine gun, one of the best ever built, if not the best. the brits made a lot during world war ii, and fired lots of shots through them, perhaps 15 billion rounds at totaled, if one source is to be believed. factor in the americans, the germans, the russians, the chinese and the japanese into all that, and don't leave out the australians, fergeddabout the french, and that is a whole lot of ammo being fired through machine guns. the great majority of which, didn't hit a damned thing except air and the ground. all those people would have done more damage to each other equipped with hatchets, and made far less noise and smoke. they are sinister as hell, and just about worthless, in my estimation.)
and, with an ar-15 trigger group of one sort or another, i can have a weapon that will fire single shots with a pull of the trigger, a burst of 3 shots with a pull of the trigger, or will fire fully automatically with a single pull of the trigger, all with the same trigger group. all i have to do is pay my national firearms act excise tax to the friendly local neighborhood office of alcohol, tobacco and firearms, pass the application and records examination by a.t.f., and find a state in which it is legal to shoot a machine gun, and i can make all the empty brass my little heart would every want.
as to caliber, i can make it in any caliber i want, so long as it will fit in an ar-15 magazine/magazine well, of any barrel length. say i want a 22 inch barrel, and to mount the whole thing on a bipod. no problem.
i buy the appropriate trigger group, remove the old trigger from a lower with two pins, and place my upper on my lower, and i am ready to go.
and, i can "customize" an ar-15 to perform any function that an rpg can perform. with the same 7.62x39mm russian round, if i like that. or, with a .223, or with a 6.8mm remington spc, or a 6.5mm grendel if that suits my fancy. if i am a real glutton for punishment, i can step up to the ar-10 platform, and do that with the 7.62x51 nato round.
all i have to do is remember to use a long enough gas tube so as not to cook the upper receiver, or to use a short stroke recoil system as now available in the ar-15.
the genius of this rifle lies in the trigger group.
you can fire single shot, bursts or full automatic fire. you can fire semi automatic only.
it doesn't require any change to the bolt, or bolt carrier assembly to do this. the bolt carrier goes back and forth, and the bolt turns to go in and out of battery with the breach and load and unload the ammo, and that is it.
and, back to the point about direct impingement or short stroke piston operation.
and, that is, you don't like something about the ar-15, ... , change it.
if you are satisfied building your ar-15 with the direct gas impingement system, or if you want to mimic the russian rpg with a gas system, by all means do so.
if you are worried about cooking the receiver and bolt group by firing too many rounds on full auto in your ersatz rpg, because the direct gas impingement system is putting too much gas into the receiver to operate the gun, ... , then simply go to a full rifle length gas piston/operating rod system, and pick from among the about 6 or 7 systems available out there. pick a short one, an intermediate one, or a rifle length system, it won't make much of a difference, the rifle will work, and pretty reliably if teethed correctly.
and, this goes across the board. you don't like a feature of the basic ar-15, change it. you can, as a matter of mechanical fact, change just about any aspect of the ar-15 without having to alter any other aspect.
you wanna change the trigger/fire control? do so, it does not alter the function of the bolt/bolt carrier or gas system in the slightest. you want to put an operating handle out on the barrel shroud, or on the left side of the receiver, go ahead, you may do so with changing the trigger group for fire control, or really without having to mess at all with the bolt carrier group.
and, that is why the ar-15 rifle design is probably the best in the world.
again, you don't like something about it? you think the caliber is a pipsqueak? build an ar-15 in .300 winchester short magnum, if you want some oomph. you can do it. how do i know?
because, people have. with just no trouble at all.
p.s. as a matter of fact, i saw an advert the other day, pitching the ar-15 platform security detail mission, requiring a short coupled hideaway discrete carry weapon. ala hk.
it featured an ar-15 with a 7 to 10.5" inch barrel, so you can wear it under your coat like the security guys do with the hk's. designed to go full auto, clip after clip, without cooking the receiver, bolt & bolt carrier via the use of a short stroke piston system.
and, burst fire, full automatic. from a closed bolt, or an open bolt, your choice on the spot. back and forth, forth and back.
the kalishnikov is a wonderful rifle and caliber. a splendid rifle. you cannot, however, do that with a kalishnikov, period. not any day of the week.
the ar-15 system, you can make it do anything you want. that's why it is the best. and, that is why designers use it, for everything.