OK, the rifle and ammo were left in a warm place overnight. The shooting shack temperature was 28F while the outdoor temperature was 20F despite sunshine. There was a 10 mph wind quartering across the range and there was a wee bit of mirage, but not enough to complain about. I carried the warm rifle & ammo into the shack, immediately fired 2 warm-up shots, and then immediately commenced group shooting, firing as quickly as I could load and aim, and not allowing the barrel to cool between shots as I would normally do.
The results speak for themselves. The warm ammo shot just as well as it did on on December 14.
I had replaced the pexiglass exit with cardboard because I got tired of errant gas checks punching holes in the pexiglass.
Well, there were no holes in the cardboard todayConclusion:
-- my cast bullet loads are worthless when the ammo is cold.
-- I'm guessing the lube (BAC) does not work right when it is cold.
-- for the time being, heating the shooting shack is the lesser evil, plus it's more comfortable.
-- but if I heat the shooting shack, then I must contend with terrible mirage in the shooting muffler. The colder the outdoor temperature, the worse the mirage.
-- so I may be out of luck until the weather warms up. I'm guessing the outdoor temp needs to be at least 45 - 50 F to bring the mirage under control.
Longer term, I may attempt to find a lube or a bullet design that is less sensitive to cold, but that could take a long time, and there may not be a simple solution. Armchair Theory About Why Cold Hurts Bullet Lube:
I have no way to prove this but here's my thinking -- when any bullet is engraved by the rifling, the metal that is displaced by the rifling has to go somewhere. In the case of a jacketed bullet, the entire bullet gets squeezed and becomes longer. Nonetheless jacketed bullets seem to tolerate getting squeezed. Monolithic copper bullets do not like getting squeezed -- it increases pressure and hurts accuracy. So the trend in monolithic bullets is to have grooves. Rather than squeezing the entire bullet, displaced copper is simply pushed into the groove. Hence grooves are good things to have even if you are shooting a coated cast bullet that does not require lube.
But what happens when a lubed cast bullet is engraved? Well, the lead is pushed into the groove. That displaces lube, which is an incompressible fluid. So where does the lube go? Most likely it squirts backwards, into the throat & neck area. Even in a benchrest gun there is still a little bit of clearance in the throat and neck, so there is room for the lube to squirt. Usually this squirted lube in the throat eventually gets melted and consumed by the hot combustion gases, but sometimes you'll end up with lube buildup in the throat, or on the neck of fired cases, particularly at lower pressures. I've had loads that would leave lube smeared on the case neck, and that's what first made me suspect that the displaced lube must be squirting backwards. Plus, if you think about it, there's nowhere else for the displaced lube to go.
OK, so at normal temperatures the displaced lube simply squirts backwards and then is consumed by the hot combustion gases. But what happens at cold temperatures if the lube is too stiff to squirt? I'm wondering if cold, hard lube makes the bullet act like a solid monolithic bullet, distorting the entire bullet? Also, it's possible that the displaced lube in the check shank, rather than squirting back into the throat, pushes against the gas check and pops the gas check off? I had experimented with removing lube from the check shank on June 30, 2016
, and at that time it did not solve the flying gas check problem, but it's possible that there is more than one cause of flying gas checks, so I may have to revisit that theory.