I've got a Franken A2-style AR that I bought used about ten years ago, older Bushmaster lower and GI upper. I've put over a thousand rounds through it, mostly 55-grain 5.56 but also some .223, and it runs flawlessly and shoots well. I've been considering going to a 16" upper and note that the shorter barrel is usually accompanied by an adjustable-length buttstock. I'm 6'1" and wear 34-length sleeves and the A2 buttstock fits me. Is there a reason to go with the adjustable version other than looks?
When I bought the rifle, I scrounged around and bought maybe a dozen surplus mags from various manufacturers and replaced the springs and followers in all of them, plus a couple of new Brownell's metal mags, plus a few Pmags. They all run, though admittedly they don't get much use. For SHTF purposes I have a pair of recent (M3, stamped "26") Pmags loaded with 55-grain Gold Dots, one of which is in the rifle in Condition 3. Is there a reason to dump the metal mags? It seems they get short shrift around here.