M16 build.
 

I'm looking to make a semi auto M16/AR-15 build, preferably a retro A1 style build (since the actual M16A1s cost over 10 grand they are out of the question). Does anyone know of the ease of acquiring an A1 upper or lower, stripped or complete? Also, if your upper is safe for 5.56 (like an A1 upper) and your lower is .223 only (say an SP1 lower), is it safe to fire 5.56x45 ammo out of the gun? Any comments, advice or suggestions on M16s are welcome as well. Thanks.

Your lower being in .223 makes absolutely no difference, all the lower does it house the mag, trigger mech and buffer spring.

Maybe my mind isn't working right (I'm operating on 5 hrs. sleep), but...5.56x45mm upper versus .223 lower? Huh?

If I were going to make an M16A1 look-a-like, I guess I would just mate an A1-style upper to a lower from any other AR-15, with the stock replaced replaced. I know I've seen A1-style uppers (with forward assist) before for sale at gun shows.

I'm saying if you mate a 5.56x45mm upper with a .223 only lower, is it safe to fire 5.56x45 ammo out of the gun?

5.56x45mm is military metric. .223 Remington is in caliber inches. They are the same bullet all in all.

I know that, but 5.56 is loaded to higher pressures, and is what the military uses in their guns. I'm saying if the lower half of the gun is for the lower pressure round (.223), is the gun safe to shoot without KBs?

I'd say it would work fine then, the difference isn't that drastic.

With the exception of the buffer tube, I can't think of any components in the lower receiver that would even be affected by higher pressure.

Anyway, I am not aware of any AR-15 lowers that are built specially for military-grade ammo and others built to handle civilian ammo (which isn't to say there's no difference between an M16 and an AR-15 lower, but those differences have to do with ability to fire on automatic, not ammo types).

As I said earlier, no difference. It's safe to interchange the ammo anyways, and the lower is not a pressure-bearing part. Even if the upper is in a completley different calibre, the lower makes no difference as long as the mag fits - 6.8mm SPC upper, for example, fits fine on a .223 lower.