Picked up my 458 and ran some rounds (factory 510gn Wunchesters with the large lead tip) through the action, same ones (without firing them) in the same position in the magazine each time. The second and sixth loaded round (both sit in the left hand side of the magazine) are the ones that give problems. The sixth can be cured easily by loading to the rear of the magazine. If the second and first were at the rear of the magazine the problem was improved but not completely eliminated. I messed around with a temporary shim in the front of the magazine and it seemed to fix the problem, but the lead was badly chewed at that point and I didn't want to destroy any more projectile points.

I noticed that when second round is at the front of the magazine it will do a quick short flip up and to the right as it enters the action (I think that it may be the belt of the second round sliding off, and jumping, the belt of the first round?) and will jam inside that part of the action (as 4seventy had said). I'll make a better quality shim and insert in the front of the magazine and then see how that works. Then again I'll probably use the Woodleighs for reloads anyway and won't have the problem, but I'm keen to see if there is a simple solution.

Summary of round condition after about 30 cycles each: the lead portion of round number two was badly chewed, next in line of chewed-ness was round 6 (which had also jammed) and then equally round 5 and 1 (none of these had jammed and I was suprised that they were marked). Rounds 3 and 4 had very minor marks on them.