Originally Posted by LouBoyd
At least one other cartridge which feeds though the AR-15 lower magazine is 2-1/2" 410 shotgun. An 18" smoothbore semi-auto upper is imported from Safir Arms in Turkey.
There are several uppers in which ammo does not feed though the lower. One is the AR-57 upper. It uses a 50 round FN magazine mounted above the barrel and shoots 5.7x28 FN ammo. It's available in pistol/SBR or carbine barrel length. It uses the lower mag well to drop or catch spent brass.
There are also several makers of large caliber single shot uppers which use the AR-15 lower. Those are available in 50 BMG, 416 Barrett, 338 Lapua, and other calibers.
There's even the PSE TAC crossbow upper for an AR-15 lower.
Lou is right . There are so many that it is hard sometimes to decide.
If you want a middle of the road caliber the 6.8 might be a good choice and another good one
that has not been mentioned is the 450 Bushmaster. It uses a 250 grain bullet @ 2200 ft/sec
and is death on hogs. Bushmaster makes the upper (It is specail built for this cartrige) and
comes in 2 barrel lengths. I also have the Tac 15
for my AR lower. all the uppers fit, and function
great on my Rock River lower and are easy to change out.
The only modification I made to the lower was an aftermarket trigger.
Lots of choices. look at as many cartriges as you can and decide based on need and usage.
J E CUSTOM