Scott,
This happened to me when first shooting Grendel. Bullets would creep forward as they slammed into battery. Sometimes jamming in the lands which would give split vertical groups on the target. Not good for pressure spikes and a problem needing fixing.
Took the expander ball out of the Forster FL die and now the difference between OD of Lapua cases sized, and bullet seated is 0.004". Before, it was 0.001" which is fine for bolt guns, but not auto loaders. Bullets don't move now.
Lee crimp dies work brass way more than they should. Their crimp dies chew the mouth of cases to get the crimp; maybe they're designed for bullets with a cannelure. Redding makes a less severe taper crimp die if you want to solve it by crimping. As for Lee dies, I recommend getting better quality dies and the more you shoot, you'll end up doing this anyway.
As for compressed loads creeping forward. Yes, I've heard this happening with long-term storage of ammo. The longer you store, the more it creeps. Gripping bullets lightly is not going to help. Unlikely to be the problem here however.
Another possible cause is lack of bullet shank contact with the case. I am unfamiliar with the 105mkz but it is on the lighter side so might not have enough shank contact to provide a decent grip. Pushing bullets further in helps, as long as you account for the increase in peak pressure. Jump-to-lands becomes more severe but can't be helped, and might not make any difference to accuracy anyway.
What is the difference between the OD of a sized case at the neck, and the same spot after you've seated a bullet? This is the grip I am talking about above. What brand of case is it?
This happened to me when first shooting Grendel. Bullets would creep forward as they slammed into battery. Sometimes jamming in the lands which would give split vertical groups on the target. Not good for pressure spikes and a problem needing fixing.
Took the expander ball out of the Forster FL die and now the difference between OD of Lapua cases sized, and bullet seated is 0.004". Before, it was 0.001" which is fine for bolt guns, but not auto loaders. Bullets don't move now.
Lee crimp dies work brass way more than they should. Their crimp dies chew the mouth of cases to get the crimp; maybe they're designed for bullets with a cannelure. Redding makes a less severe taper crimp die if you want to solve it by crimping. As for Lee dies, I recommend getting better quality dies and the more you shoot, you'll end up doing this anyway.
As for compressed loads creeping forward. Yes, I've heard this happening with long-term storage of ammo. The longer you store, the more it creeps. Gripping bullets lightly is not going to help. Unlikely to be the problem here however.
Another possible cause is lack of bullet shank contact with the case. I am unfamiliar with the 105mkz but it is on the lighter side so might not have enough shank contact to provide a decent grip. Pushing bullets further in helps, as long as you account for the increase in peak pressure. Jump-to-lands becomes more severe but can't be helped, and might not make any difference to accuracy anyway.
What is the difference between the OD of a sized case at the neck, and the same spot after you've seated a bullet? This is the grip I am talking about above. What brand of case is it?