For years I heard stories about gas checks coming off in flight, but never experienced it personally, so I was skeptical. Then last year a Pacnor 6BR starting slinging its checks and took out my chronograph. That got my attention!
Still, none of my other guns were slinging checks at that time, so I thought it must have been caused by something peculiar to that particular barrel. More on that later.
Recently, my Pacnor 7 BR started slinging about 10% of its checks. Its checks haven't hit the chrono (yet) but they have hit my pexiglass muffler window. The pexiglass window is probably catching errant checks that might have formerly gone un-noticed.
At this point I have some half baked theories about gas checks coming off, but don't have a lot of confidence in my theories. The errant gas check problem tends to be random and mostly infrequent, making it difficult to devise a test to "prove" whether or not something fixes the problem. What I am going to do in this thread is make note of all incidents of gas checks coming off, and of any observations related to those incidents.
-- 2 checks hit the pexiglass window out of 22 shots fired
-- at least one de-checked bullet landed in the main group. Not sure about the other de-checked bullet.
-- this may or may not be relevant but accuracy of the load was very poor.
-- bullet bases were 0.2850" - 0.2855", perhaps larger than ideal for the the throat, which is closer to 0.2840" - 0.2845".
-- the barrel is getting "old," with probably 2500 or so hi-vel rounds through it.
-- the check was a perfect fit on these bullets as they dropped out of the mold.
-- the bullets had been swaged at 47,000+ psi, so even if the check fit were not already perfect, swaging definitely made it perfect.
-- the gas checks had been annealed.
-- 2 checks hit the pexiglass window out of 104 shots fired
-- at the time I blamed myself for shooting a bullet whose check had become stuck in the neck when I ejected a live round and the bullet stuck in the throat. Even so, that did not explain 2 holes in the pexiglass.
-- accuracy was decent, averaging 1.42"
-- while I did not record the as-sized check diameter, the fact that the alloy was unusually hard (40 - 45) may have resulted in more springback after sizing.
-- an errant check took out my chrono. An autopsy of the chronograph verified that it was hit by only the gas check, not by a whole bullet.
-- subsequent autopsy of the wooden box holding the chrono found many more 6mm checks embedded in the wood, but no checks of any other caliber.
-- subsequent testing revealed that nearly 100% of checks were coming off in flight, at close range (15 feet or less).
-- by this time the barrel had 1500 or so hi-vel rounds through it.
-- experimented with no lube in the check groove, based on the theory that hydraulic pressure might be pushing the checks off the shank.
-- 100% of checks still came off in flight.
-- the entrance to the throat was chamfered with a custom reamer
-- after the throat mod, only 1 out of 6 checks came off in flight, a significant improvement.
-- the problematic 6BR barrel was set back and rechambered to 6mm TallDog with a deliberately sloppy throat entrance.
-- zero checks came off in flight.
-- but accuracy with the sloppy throat was poor.
-- the 6mm barrel was set back yet again and cut with a "tight" throat TallDog. But the entrance to the throat was lightly chamfered.
-- zero checks came off in flight.
-- the "tight" throat was more accurate than the "sloppy" throat, but still unsatisfactory, I began to suspect that the barrel was simply "shot out."