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I bet your seater die is not aligning the bullets well enough. Either that or the die is too far down and you're applying excessive crimp. If you're not crimping, then it's bullet misalignment.the expander button button measured just over .219, I ordered some boat tails and will see if that resolves the issue, if it is still not working I will start messing with the dies.
What is your thinking on this as far as neck length? If he's not crimping, I'd be shocked a little longer neck would cause this.Are you trimming all your cases uniformly to spec after resizing? When seating your bullets, if one neck is longer than the neck you set your die to, the shoulder will deform.
You don't need to use an expander button at all.. you can use a Lyman M die to just allow the bullet to be seated without deformation instead, very often you must use a like die as when using plain base lead projectiles and the like.
I have RCBS dies and have loaded lots and lots of flat base bullets with out problems. Me and about a million + others. That company makes good products and they stand behind them.
The first thing i would have done is call the RCBS tech line and talk to a professional.
Chamfer your neck a touch more than you would with boat tails. Don't use overly work hardened brass that has been fired to many times.
Set it up as per the manufactures instructions.
Try a different brand of brass. Try a different type of bullet.
All easy stuff.
You can't use those for compressed loads so it'd suck for crushing loads.One and done.
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They say to avoid using compressed loads with that die.I have a Forster UMSD and i have loaded thousands of compressed loads of H4895, Varget & AR-Comp. Still works fine. Me and many, many others.