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Can someone give me a "for dummies" rundown of what you are doing with drill bits in here?
I have built 7 ARs and have never heard of this.
I am not criticizing at all, just genuinely curious about what you guys are doing here.
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As stated, it for the release of the hammer off the auto sear in full auto rigs.
Ideally, you looking for a gap of .084" from the face of the carrier, to the face of the barrel extension when the hammer is released.
Hence as the carrier is just about to touch the face of the barrel extension, with the correct hammer release timing, the round is fired before the carrier has a chance to bounce back off the face of the carrier, and block the firing pin (in the case of the gap being too small and the buffer is not controlling bolt bounce instead).
If the gap is larger, then it comes a point of which you are either going to speed up the cycle rate, or if the gap is way too large, then the hammer is going to strike the FP while the back of the carrier still has the FB blocked.
As for gauges, standard drill bits work fine, since you are just checking to make sure that the timing is around the .084" range.
So #44 (.086") works great as a no go gauge, and #45 (.082) work great as a go gauge.
http://www.gearhob.com/eng/design/drill_eng.htm
As for adjusting timing, that comes down to either mod'd the auto sear, or the catch shelf on the bottom of the carrier instead.
Now this gets a little more complex when you have several uppers that you plan on pulling a single RR lower instead.