Firearms Stack Exchange Archive

AR-15 - Failure to Feed - Rifle Length Gas System with Standard Buffer/Spring

I have a CMMG AR-15 with 18’’ Barrel and rifle-length gas system. The rifle came with a carbine-length buffer tube. I frequently get FTF (similar to this) where the BCG scrapes over top of the round. I believe is is due to this seemingly mismatched combination of gas system and buffer system as it appears the BCG comes back too fast or never retracts fully.

My question is would a heavier buffer and/or weaker spring help? Should I prioritize replacing one before the other? or Replace both? Are there other suggestions to troubleshoot and fix the issue?

Answer 211

A carbine-length gas-tube actually experiences much higher pressure than a rifle-length gas tube, and as such, the carbine's buffer spring is heavier than the rifle's. Your BCG is not moving back far enough, and as such, you're short-stroking and not stripping the next round off the magazine.

Change the buffer, buffer tube and buffer spring for a rifle buffer, buffer tube and buffer spring, and you should be good to go! Really you're only looking at a $40-or-so fix at most.

Graph comparing the chamber pressure of different AR-15 gas systems

Please see http://ar15barrels.com/prod/operation.shtml for a much more detailed description of how the gas system works.

Answer 208

The longer the bullet stays in the barrel the more gas that builds up to push through the gas system. I believe the buffer is too light (since it’s expecting the lower strength of a carbine length gas system). A weaker spring may slow things down a bit, but with the power provided by the rifle length gas system, I don’t think it’s necessary.

My suggestion: get a new buffer first then a new spring if absolutely necessary. (Neither are extremely expensive, so replacing both at the same time would save some time.)

Answer 369

Short stroking and the resulting failure to feed is usually indicative of a gas problem. Not usually buffer problem. Before swapping buffers and springs, check to ensure that the gas key on the carrier is installed and properly staked, and is not suffering blow by.

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