And if so what kind.
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And if so what kind.
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My friend had issues with the O-ring. It caused the extractor to have too much tension and it wasn't ejecting the brass after being fired. He took the O-ring off and hasn't had an issue.
I wonder if to much tension would keep the round from feeding by preventing the round from locking into the bolt head before the round chambers.
Since you are talking about this in another thread I would have just kept this post there. Why don't you explain exactly what type of upper you are using now, barrel length, etc...so we can get a better picture of what's happening. To be honest I am confused after reading your initial post.
It is entirely possible that the extractor could be so tight as to not grab the rim of the cartridge as it should. I would remove it and then test fire the weapon.
Usually if it's going to cause problems it'll be failure to feeds. The rounds will make it most of the way to the chamber but the bolt won't lock. If using a 5 coil extractor spring, black extractor buffer and a O-ring you should be sure that the ejector and spring and buffer spring are up to the task. This is one of the reasons why I replace buffer springs when they get down to 10 1/4" instead of waiting until they are right at the bottom edge of the spec at 10 1/16". I use Brownells standard rate chrome silicone springs which start out at 10 1/2" when new. Also be sure to keep the ejector lubed up and make sure it's moving freely. Too much extractor tension can over power the ejector and you'll get poor or no ejection.
On the rifle no and no
On the BCM assembled upper yes and yes.
Today I'm going to place my BCM upper that has last had feeding issues on my another rifle lower I will test fire to see if I have failures to feed. If I have failures I will than remove my O ring and retest.