BCG dissection

[LRRPF52]

The AR-15 TDP has some very specific call-outs for dimensions, materials, processes, and finishes in the BCG so that you have consistent and long-lasting mechanical articulation and gas system efficiency.

I’d like to see BCG manufacturers adopt the armorer’s tools for their own QC and certify them at least dimensionally to those specs, but this costs money due to having trained personnel running the checks with the proper tools.

Bill A. really turned me onto those aspects of the TDP many years ago, enough so that I made a diagram of a cross section of the BCG:


Notice the 3 different critical Inner Diameters within the bolt bore in the carrier.

1st is for bolt alignment so it consistently goes into the barrel extension.
2nd is to compress the gas rings around the flange, forming part of the expansion chamber seal until telescoping allows it to vent out of the holes seen through the ejection port.
3rd is the bolt tail aperture, which has to be tight to prevent gas escape out the back of the carrier and down into your FCG.