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Old August 15, 2012, 11:53 AM   #13
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Join Date: November 8, 2007
Posts: 2,001
"Step-down" crimp

The crimp on the right seems about right for a jacketed bullet. As somebody has already written, the idea is to turn-in the case mouth until it touches the bottom of the cannelure, but no further.

HOWEVER, with the advent of some of the heavy-recoiling cartridges like the .500 and .460 S&Ws, there has been some rethinking of the crimp so as to keep these revolvers from walking the bullets out of the case mouths and tying up the cylinders. Speer calls these extra-heavy crimps "step-down" crimps, and makes SOME of their bullets with flat-bottomed cannelures to accomodate them. The case on the left has what looks like a step-down crimp.

For a step-down crimp, the idea is to make the case mouth start at the front of the cannelure and lie flat for a short distance in the bottom of the cannelure until it reaches the back, then expand back to the uncrimped case diameter soon enough that the bullet itself is not swaged/distorted by the crimp. That is definitely hard on the case mouths, and will reduce case life by leading to early splitting. And, with some dies that have sharp edges on the crimp ring, it will scratch brass and may scrape-off nickel plating.

So, it is not a good idea to use a step-down crimp unless it is needed to keep the bullets from moving under recoil.

In a .357 Magnum, I would not think that a step-down crimp would be needed, unless perhaps for ammo to be used in one of those new ultra-light guns that is designed for concealment.

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