Question about AR lowers

[General Geoff]

My understanding is that a standard .223 lower is capable of accepting pretty much any caliber upper that has the same case length as .223/5.56mm. If that is the case, then the lower receiver has essentially no bearing on what is safe/not safe to fire through any upper that may be attached to it, correct?


I'm looking at buying a few stag or rra lowers, as they can be had dirt cheap, and then I'll build up the rifles later with barrelled uppers (which from my understanding can be shipped directly, as they're not considered firearms).

Am I correct in my understanding? Sorry, I don't have much experience with ARs yet (but I plan on rectifying that soon!)

 

[briansmithwins]

You are correct. The lower doesn't have any influence on what round is acceptable for the upper, as long as the cartridge fits within the 5.56 size magazine. For example, for a .7.62 Nato AR, you do need a different upper. BSW

 

[RockyMtnTactical]

Actually, with a 7.62x51 you'd need a different lower and upper.

 

[CWL]

You aren't even limited by the size of the lower receiver.

There are upper variants that are bolt action and feed directly from the top without needing the lower receiver. Most common example would be the .50BMG uppers.

 

[RockyMtnTactical]

Right, but with a .50cal upper, you don't feed it through the AR15 lower magwell... So, with larger calibers like .243 or .308, you have to have a larger AR10 lower.

 

[General Geoff]

Actually, with a 7.62x51 you'd need a different lower and upper.

Yes, but as I stated in the OT, 7.62x51 has a longer case length than 5.56 and thus the lower recever does matter.