[AK103K]

I was just thinking, if they can come up with a .50 caliber upper for your standard AR lower, and a belt fed upper(Shrike), why not a mag fed pop on .308 upper? Why couldnt it be fed from the side ala the FG42 using cheap FAL mags. Seems doable to me. :)

[Futo Inu]

Hmmmm, good point...interesting.

[Joe Demko]

If New Zealand could turn Lee-Enfields into machine guns, then what you describe is child's play by comparison. There is only one question:
Is there enough of a market for the product to make it profitable?

[Black Snowman]

Probably due to the availability of AR-10s, FALs and M-1As that already seem to fill this nitche. The cost of a complicated upper like that is likely to be pretty close to the cost of an AR-10 and certainly more than a parts FAL. At least close enough to make manufaturing it not particularly profitable.

Now if you want a more exotic cartridge you're more likely to get your money back out of the tooling. Plus, I'm not sure I want to shoot .308 out of something as light as an AR-15. That could get pretty brutal :)