I'm confident that they are hard enough and the right size (the first few will shoot straight until, I think, the barrel leads up) and that a gas check would probably fix the problem at the higher velocities.

Would an undersize projectile (not gas checked) cause barrel leading if the velocity were otherwise within the range appropriate for that particular lead alloy hardness? I guess any gas sneeking past would tend to melt the lead wouldn't it?

There's a lot of info out there - another guy I have spoken to has advised that I steer clear of the conventional pistol powders in the larger rifle cases (especially the straight walled ones) since they do occassionally ring the chamber, even if no filler material is used. Trailboss is different of course since it fills the case - the empty space and faster powders cause the problems. I can understand the dynamics since the same thing can happen if gas is ignited within connected enclosures that don't have appropriate sealing arrangements - the effect is called "pressure piling". The initial explosion will compress the gas at the other end until it do ignites and then sends a pressure wave back to the original expolsion source. "Pinging" is petrol engines is also an example of this.