They're sized to .358 and can't pass through the cylinder unless I smack 'em with a dowel.

Like I mentioned, the other loads shot straight and true, just not these.

I will keep an open mind, but I am leaning to the no gas check on a checked bullet design at moderate speed as a contributing factor.

All the loads I put through it were the same bullet, sized the same made from the same alloy. The magnums with the gas checks installed were fantastic as were the +P loads w/o the gas checks, but boosting the speed and no check didn't work out so well.