confused.22lr upper, 5.56 lower

[BULLET8071]

so i sold my complete spikes tactical dedicated 22lr upper and lower a year ago due to financial discrepancies. now heres my question...

i just purchased a CMMG dedicated upper and would like to match it up with my lower which is a Sabre Defense xr-15, but is marked 5.56..i want to just match it up so i can complete this rifle, but, am i going to have to deal with legal issues since it IS marked 5.56? ive done my homework and researched, but it says nothing about marked lowers if it doesnt specify "multi caliber"...(i know i can just get another lower reciever, but this lower has been sitting in my safe for a few years now, and i dont feel like spending a few extra bucks on a lower if i dont have to)

i did speak to my buddy in the LASD, but he's just as clueless as i am with the question i asked. can people please chime in?

[Grumpyoldretiredcop]

The caliber designator on the lower is irrelevant. What the weapon actually chambers is important. If you install a .22 dedicated upper (or a conversion kit in a 5.56 upper) on your lower, it's a rimfire rifle no matter what the receiver says.

[BULLET8071]

thanks for the replies guys. it cleared up a lot of my confusion. i guess i'll be able to save a few bucks from keeping this lower!

[OutlawStar]

Dumb question but still relevant; could you also put a larger caliber upper such as .308 onto a .223/5.56 lower? I saw Rock River arms' .308 uppers but they claim you can only use the uppers on special lowers. Is this because of magazine box size?

[Cokebottle]

Dumb question but still relevant; could you also put a larger caliber upper such as .308 onto a .223/5.56 lower? I saw Rock River arms' .308 uppers but they claim you can only use the uppers on special lowers. Is this because of magazine box size?

.308 upper is not compatible with a 5.56 lower. The .308 takes an AR-10 lower.

The 5.56 lower can accommodate .223, 5.56, 9mm, .22lr, 6.8, 7.62x39, .45ACP, .50 Beowulf, and I'm sure a few others.

The main issue probably is the cartridge length... 45mm max.