BCG dissection

[Scouttracker]

I found this great video from
School of the American Rifle.
Its about 14 minutes long, but worth watching , it's called.
FAIL ZERO AR-15 BCG AUTOPSY

[dpete]

Link?

[mdram]

https://www.youtube.com/watch?v=CrJl3LThLFw

[Scouttracker]

https://www.youtube.com/watch?v=CrJl3LThLFw

Thanks I was having trouble posting the link. I'm technically illiterate.

[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.

[montana]

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.

BCM created their own air gauge tools to test their BCG's 3 bores..https://www.youtube.com/watch?v=BaFOZAwspYI

[Rivercityjeff]

Thanks! I went down the rabbit hole with a bunch of their videos.

[Vin]

BCM created their own air gauge tools to test their BCG's 3 bores..https://www.youtube.com/watch?v=BaFOZAwspYI

While a pressure check can tell you the current gas efficiency of your BCG, tail, rings, etc - what it cannot tell you is its wear as a predictor of failure or indicator of need for replacement. Physical measurements with pin gages and micrometer are the best way to perform PM checks of these parts.

[montana]

While a pressure check can tell you the current gas efficiency of your BCG, tail, rings, etc - what it cannot tell you is its wear as a predictor of failure or indicator of need for replacement. Physical measurements with pin gages and micrometer are the best way to perform PM checks of these parts.

Pin gauges are not fool proof as some poorly machined bolt tail bores will pass the pin gauge test, yet when viewed by a bore scope and back light, the bore hole can be out of round that an air gauge would find..The same principle for using a taper gauge for the firing pin and cam pin hole with the bolt..

[LRRPF52]

Pin gauges are not fool proof as some poorly machined bolt tail bores will pass the pin gauge test, yet when viewed by a bore scope and back light, the bore hole can be out of round that an air gauge would find..The same principle for using a taper gauge for the firing pin and cam pin hole with the bolt..

That’s an excellent point, especially if that bolt tail aperture was oval. An oval shaped hole would allow gas leakage where it isn’t supposed to happen.

Bill A. pointed out that obviously-inefficient AR-15s would exhibit blatant gas leakage into the FCG area.

He got a lot of training on this when at UK MoD, including from Eugene Stoner, who came out to visit them and discuss various aspects of the design that most aren’t aware of.

Certain elements of the UK have been using AR-15s since the early 1960s, so they have a long history of armorer and direct support for the AR-15 family of weapons.

[LRRPF52]

Another great video from School of the American Rifle:

[video=youtube_share;Yfs1Czm8iiE]https://youtu.be/Yfs1Czm8iiE[/video]

Notice how the gimmicks are way downstream and the BCGs that adhere to the TDP have the best gas system efficiency.

[Clintster]

Both were well worth watching. Really found it interesting in the first video the mention of lose primers going into the gas key port... just happened to me about a month ago so maybe more common than I thought it would be.