So if I were to order a BCM FA BCG (which I will soon), it'll come with the upgraded insert and spring installed, but the O-ring separate? The O-ring should only be installed if I'm having extraction issues right?
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I use a BCM FA BCG without the O-ring in my LMT MRP 16" (which is a mid-length) and have had zero issues in the ~2000 rds fired most of which were shot during a Pat Roger's 3 day class.
I agree with others that for a mid-length gas system that it is probably not going to be needed . IF you are experiencing problems that are related to insufficient extractor tension or want to diagnose if that is the cause, then the o-ring should be useful for diagnosing/fixing the problem.
Also, if you think about the function and design of the extractor system, it makes sense that there is an upper limit in terms of spring pressure in addition to a lower limit. Moreover, this operating window of spring pressure limits will change from gun to gun.
If you don't mind me asking, which middy did you purchase?
I have a middy and have run it for some time with the BCM upgrade minus the o-ring.
-RD62