Clip = something that holds rounds together inserting into a magazine
Magazine = "ammunition feeding device," as in the thing that holds all of the rounds to be fed directly into the chamber
Shells = shotgun shells ONLY...NOT .22 rimfire or centerfire rifle/pistol rounds
AR = "Armalite Rifle," not "Assault Rifle"
Bushmaster XM-15 = This is an AR-Type rifle!! (for some reason, many people seem to think it's something different)
Grains = when referring to loaded ammunition, "grain" is almost always referring to the weight of the bullet (Federal 55gr. FMJ .223 ammo = 55 grain bullet). When referring to reloading ammunition, you need to specify whether you're talking about bullet weight or the weight of the powder charge (I load my .45 ammo with 4.7 grains of Bullseye powder and use 200 grain bullets).
MOA - Minute Of Angle (a measure of angular displacement equal to 1/60 of one degree, usually used when discussing group sizes - 1 MOA is roughly 1" at 100 yds)
Quad Rail, Forearm, Forend, Handguard, etc... = all referring to the same item, though there are MANY different variations and types. They're all, essentially, a barrel shroud
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