The McFarland ring can be tight as hell to the carrier when the bolt is installed in the carrier, which will cause bolt movement problems, hence short stroking issues.
Bluntly, would save the McFarland ring for when you have major wear in the bolt/carrier that standard rings will no longer solve, and trying to get more service life out of the B/C instead.
As for the test to see if the rings are still good, just pull the bolt all the way out, then aim the bolt straight up. If the bolt drops down under it's own weight to the closed position, then the rings are shot. I state this, since if the rings past the test, and you have having problems, then look elsewhere for a gas leak instead. The leak could be the key against the bolt, the gas tube to the FSB, or even the FSB to the barrel. Also, could have some nasty binding issues, such as the key too wide for the upper receiver, the gas tube not indexed correctly with the key, or even hammer wedging with the tail of the hammer too long.