What Happened?

[Grumulkin]

It's not really perplexing. All 8 rounds were borderline low on the pressure and the 8th just happened to be a bit lower.

A similar thing happened to me once when working up a 454 Casull load in a Freedom Arms revolver for which there was no published load for the rifle powder I wished to use. One out of the ones I shot locked up the cylinder. No more problems occurred when I increased the load.

 

[Old Shooter]

I think it must be something with that individual case. Either the powder was extremely damp with cleaning compound and thus did not ignite, or the flash hole was totally plugged with cleaning media allowing the primer to fire and push the case back with just enough remaining force to lodge the bullet in the forcing cone. If the powder ignited easily and did not appear to be contaminated upon inspection the ignition flame from the primer must not have reached the interior of the case. But I could be wrong, my wife says that all the time.