Wait. Do I understand that ANY brand of the AR class of weapons has a lower that can be paired with any of the aforementioned calibers? So, like, a lower from a .223 can also be used with the upper with a .308 caliber with no major modifications? Wow! So all one would need to make the "conversion" would be the complete upper receiver and of course, the appropriate magazine?
What a concept!
The Lower Receiver for the AR15 is the part that BATF classifies as a “firearm”.
There are a whole world of available uppers and conversions for that “firearm”.
You are (sort of) correct. The AR-15 platform is basically Lego for guns. The only caveat is that there is a limit to the length of cartridge. A .308 AR-15 is possible, but would not function as a semi-auto. (Indeed, there are .50 BMG uppers for the AR-15, but they are bolt action.) If you need a semi-auto .308, you’d need to opt for an AR-10 platform rifle instead, which is the gun that the AR-15 was “shrunk down” from. Once you have the lower, the sky is the limit. Pop out two pins to switch to any compatible upper in any caliber under a cerain cartridge length.