Due to friction, a tighter carrier would cause the oring to be pushed back farther when the bolt is at rest. If the vent hole is close to the oring , then it could leak. When you use the proper carrier, the oring fits properly and doesn't get pushed back so it doesn't expose the vent hole and leak. However, low pressure causes the oring not to seal as tight against the bolt stem. This is normally not a problem unless the oring is close to the vent hole. A loose fitting oring on the edge of the vent hole may allow some air to creep by. I think there is some wear in your assembly. probably on your sear. Also, check that your rail bushing is in place. It probably is, because there would be other issues if it was missing.
Originally Posted by TheMedievalMan