Our two favorite firearm info straight-shooters, Brownells Gun Techs Caleb Savant and Steve Ostrem, have some tips on why your AR-15's bolt won't close or, to use the technical term, "go into battery." It's a pretty common problem, and guess what? There are multiple reasons why an AR bolt doesn't close. REASON #1: An overloaded magazine - 31 rounds in your 30-round mag. Just because you can shove an extra round into the magazine doesn't mean the bolt has enough oomph to overcome the mag spring's pressure and strip the round out of the magazine. REASON #2: Packing grease in the chamber. This is common on brand-new guns. Overzealous application of rust preventative by the manufacturer, and maybe in your eagerness to hit the range you forgot to do a thorough cleaning. REASON #3: A weak buffer spring - common to old rifles with a LOT of rounds down the barrel. Buffer springs are easy to replace. REASON #4: You're "following down" with the charging handle instead of just pulling it to release the bolt catch and letting it go. The AR-15 is designed for hard use, so letting the bolt smack home won't hurt the gun. REASON #5: Reloaded ammunition with improperly resized cases. An oversized case won't chamber!
