You know, sometimes I take firearm knowledge as a given. I have to remind myself that not everyone keeps permanent tabs open in their browser to at least two different gun forums at any given time...
Yes, the P90 fires the 5.7, they also sell a handgun that fires the same round to devastating effects. It is brutal, high speed and great for close in work. The round has no weight though and doesn't carry it's energy very far at all.
There are several companies making .308W/7.62N AR Pattern rifles. The military is even switching over to a 7.62N AR Pattern rifle for their Designated Marksman weapon. They are common as all get out and readily available, though only one company makes the AR10 and that is Armalite. Everyone else uses different nomenclature or they are going to wind up owing Armalite a truckload of money since it is their terminology...
Other ammo choices for the AR style weapon that will have more punch than the standard 5.56N round would be .50 Beowulf, .458 SOCOM and 6.5 Grendel. All very good rounds and capable of taking down a black bear.
As for the AR style action needing replacement, all the friends I have that are in the sandbox or have been say it is generally the REMFS that have the problems because of improper weapon maintenance. Shooters rarely run into a problem. And anyone recommending a piston system like the HK416 needs to consider this, the SEALs bought a whole bunch of HK416 rifles. And removed them from service shortly thereafter for undisclosed problems. The problem arises from how the action is manipulated by the operating rod, it rocks in the upper receiver and wears out the action channels. And shortly after that, serious failure. The AR system is simply not designed to function as an operating rod action. It works best as direct impingement, as Mr. Stoner designed it.
If you want an OpRod gun, go with an older design like the AR180 or one of the newer weapons like the XCR, SCAR or the Bushmaster ACR. All of those run very very nice, I have gotten to play with all three and they are pretty fun, though the SCAR has a sharp spot between the receiver and the stock that will bite your cheek if you're not careful.
There are lots of problems inherent with caseless ammunition, but it can still be done with something of the format of the 120mm M1 Abrams rounds fired from the M256 / Rheinmetal gun. The case burns up in the firing but the endcap is discarded during cycling. This would keep the action mechanical, but mechanical actions are going to have a problem in 0G. Unless of course it is similar to the chain drive on the 25mm Bushmaster used on the Bradley, LAV, etc.
I would think something like the
Metalstorm electric firing system would be extremely reliable for 0G environments, but you are back to needing protection from EMP bursts. But if EMP is a worry, it will be shielded, its not that big of a deal considering a non-functioning weapon is the alternative.
Holy Crap, just realized I wrote a book. Sorry about that gentlemen, I really dig firearms so I get carried away.
Take it easy,
-V