Marty, yes sir, you sure can. 223/5.56 mags will fit and function just fine with 300blk. Doesn't matter if it's lower or heavier grain. If you have a AR Chambered in 223/5.56 I'd marked the mag you plan to use 300blk in though so you don't accidentally use it in the 223. Quite a few incidents where people accidentally chambered a 300blk in a 223 and damaged their firearm. Other than that it'll work just fine. Hope this helps.

Yes you can, but please don't make the mistake of chambering the 300 in a 223 rifle.

yes you can.