Well, that's annoying. 257 Roberts and Barnes TSX.

[A382DWDZQ]

Was also going to say that it might not be a bad idea to back the charge off a few grains once the neck tension is fixed.

 

[SloppyJ]

1000% a neck tension issue. You need to go with a smaller bushing, anneal it and resize, or do something else. Like above, I've never given a second thought to where the case mouth fell on a cannelure and it's never made a difference.

Think about how long the neck is, that entire neck should be around the same diameter so the leading edge of the case mouth is not the only location it has to "grab" the bullet.

What's the OD of the neck without a bullet seated compared to with a bullet seated? That's your neck tension. I have stuff i run very little neck tension on and have no problems. Like 1.5thou so you've gotta be damn near 0.

Since you necked it down from a different cartridge, I'm betting it's just not small enough.