Og has his evil black rifle all put together and is lacking only time to get it to the range.
A discussion in his comments section brought up some good points about AR magazines.
I know I harp on this constantly, but magazines are wear items. Further, they are the weakest link of any weapon that feeds from detachable mags, if only because they are easier to drop and lose than the rest of the gun, so it's only common sense to have a bunch of 'em.
Until a few years ago, if your rifle was an AR-15 (or other type that ate out of STANAG magazines), the easiest way to do this was to buy a bunch of GI surplus aluminum mags. It's still a pretty inexpensive solution to the magazine problem, but it can have its downsides.
Left fully loaded for extended periods of time, the spring will push the column of ammunition against the feed lips, gradually spreading them apart. Your first indication that this has occurred will be the occasional double feed. If these are ignored and the magazine left in service, you may get a chance to see a "volcano", as the lips get spread enough to allow the magazine to regurgitate three, five, or even a whole magazine's worth of cartridges into the upper receiver.
Also, GI magazines have been loaded by GIs, who are given stripper clips of ten rounds and a handy little adapter to get them from the clip into the mag. If the GI is a lazy GI, he might have braced the top round in the clip against the edge of a handy table and the magazine baseplate against his tummy and leaned against it to "ZZZIP!" load ten rounds really quick-like, apparently without considering that this then allows the feed lips of the aluminum magazine to crash into the edge of the table with his body weight behind them.
While this will give him a splendid chance to practice his "SPORTS" malfunction clearance drills, it will likely not make you happy when you purchase the surplus magazine he has borked.
This can be totally avoided by buying new magazines. (And not doing these things yourself, of course...)