In my conquest to build an AR the right way, I was finally able to order and received a BCM bolt carrier group. This was, I think, the last piece to replace cheap parts in my carbine.
When I got it and installed it, the first thing that jumped out at me was how tight the gas rings were. I had to remove and re-insert the bolt into the carrier like a man. Pleasant surprise.
The second thing that jumped out at me was how freaking tight the extractor pin fit. Impossible with the supplied O-ring (I stashed it after reviewing parts diagrams in a mild panic to see if it *had* to go in there, heh).
The third thing I noticed was that when I rode the charging handle forward all the way, the bolt would not lock up under the pressure of the action spring.
All this after a healthy application of Slip EWL (liberally coated, sat for a day, wiped, reapplied).
Took it out and shot it yesterday to verify function. Was pleased that it threw the brass just to the rear edge of my peripheral vision consistently--Federal XM193, American Eagle .223, Federal XM855, and Monarch steel-cased .223. I did have 2 short strokes on the Monarch-steel-cased .223, but when it did shoot it had the exact same ejection patterns as anything else I tried.
After I got home and during cleaning, I noticed a few interesting things:
* bolt fit inside the carrier was still tight but not as tight as it was
* extractor pin was still tight but no longer challenged my manliness
* riding the charging handle all the way forward would now result in the pressure of the action spring sending the bolt into batter
The obvious answer is that the BCG has to break in, but the direct question in this post is: how long does that period usually take?
For a full picture of what's going on here, I'm running a 16" DD barrel with a carbine-length gas system, brand new BCM BCG, blue Sprinco action spring, and H3 buffer in what was a stripped DPMS upper. Can provide more specs on request, but I don't think I missed anything relevant to the BCG going back and forth.

