There is no mass movement in the U.S. Military to replace the 5.56. Curiously, the .308 is not used as much because it lacks the extended range and power needed to be effective in current theaters of operation. The Mk-11 (SR-25 .308) and MK-12 (18" barrel 5.56) are used depending on situation. The .300 Win Mag and .338 Lapua have proven to be the weapon of choice for ranges beyond the capability of the 5.56.
Having said all that, I had a Dissipator for a couple of years and I liked it BUT...I put a rifle length rail system on it and viola. I made a heavy barreled (read that varmit barrel weight) carbine that was so muzzle heavy that I eventually took the rails back off and sold it. Here is a nutshell summary of my experience in building / owning way too many AR's:
16" M4 type barrel - very handy size and weight, good looks, less expensive barrel, least accurate of the short barrel versions although some barrels are exceptional (my experience anyway)
16" Dissipator - fine if you like either plastic handguards or $400+ ultra light rifle length rail systems, shoots just like an M4
18" so called "mid length" systems - great compromise although quality 18" barrels are like buying gold
20" standard rifle - decently light but harder to manuver, classic visual appeal, better ballistics by a little bit, longer sight base gives better accuracy with iron sights, removable carry handle and scope gives a very versatile gun
20" National Match High Power (floating forend) heavy profile barrel (under the handguards), accuracy on par with the best Varmit guns - out to 600 yards, great with NM iron sights or removable carry handle and a scope for fun, way to heavy to carry around much
20" heavy barrel (so-called Varmit barrel) - wonderful accuracy with a scope (depending on how much money you put in the barrel) but weight and handiness are sacrificed, like the NM, it needs a rolling cart to carry it around
24" barrel - "Varmit" barrels are like the 16" gun barrels on the New Jersey, I saw no advantage over the 20" barrels
16" "Lightweight" barrel versions - better than the M4 weight wise (by a little), some barrels very accurate (some are not), great compromise if properly built
16" heavy barrel - great accuracy, you lose a little ballistically, better with a mid-power scope, front heavy especially with an M4 stock
16" 6.8 SPC II - great power using Silver State Armory ammo, more expensive, good accuracy but not intended to be a tack driver or long range shooter. The 6.5 Grendel is the long range tack driver, but you are locked to a proprietary cartridge. Take your pick. Never tried a .50 Beowulf or some of the other exotics so cannot comment but some exotics use the .308 size receiver
.308 versions - different animal as has been said, unique lower receiver needed, no interchanging with 5.56 uppers, the ones I've owned and shot were not anywhere near as accurate as a quality bolt gun and generally a bit more expensive - IMHO anyway (remember a great rifle shooting junk ammo is hardly better than a junk rifle shooting junk ammo!)
.50 BMG upper - what can I say? Not for the feint of heart or thin of wallet
There. Have I got you even more confused? Just build one of each. It's a sickness, don't cha know?
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