I bought a bunch of different bullets for this rifle and none of them were short enough.
Near as I can figure out the rifle is supposed to have a 1 in 20 twist. Supposed to be fast enough to stabilize.
Seat them deeper to fit....or seat your magma jobbys OUT until the lever closes with just a hint of resistance. Keep in mind they should function even if the chamber is dirty. Reduce your load accordingly and work back up to your desired speed.
Give them a slight roll crimp, wherever it may be so they don't push into the case under recoil in the magazine. The crimp groove location is secondary to jumping into the lands. Cast rifle loads detest jump...especially if the throat is cavernous and you're not making minimal land/throat contact.
It sounds like a combo of undersized (to the throat) and unbalanced (unstable) as the bullet slams unconcentric to the bore upon ignition. This gets worse with jump as the bullet deforms even more as it hits with the point off center upon ignition. Fill that "freebore" with the bullet.
A cast load that's truly undersized for the BARREL will lead the [bleep] out of stuff; especially if hard and even more so with a [bleep] lube. I'd be sure to check the barrel to make sure that isn't the case before firing more rounds down it. If its clean and the rifling is easily discerned, the bullet fit/jump in the throat is the culprit. If you can't see rifling or see globs/patches of layered lead, the groove is too large for the bullets you are using and it needs cleaned.
