.357 Mag rounds growing by 0.004-0.005 - acceptable?

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IMtheNRA

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I tested the crimp by firing 5 rounds and measuring the sixth round in the cylinder before and after. 8 sample rounds grew by 0.004 to 0.005

The load was Winchester 158-gr JPH and 7.4 grains of VV N-340, CCI SP primer, in a R-P nickel plated case. COAL was 1.580-1.590, which is quite a bit of variation due to the uneven exposed lead portion of these not pretty but cheap Winchester bullets.

Does this reflect an acceptable amount of crimping, or an indication that I may need to adjust the die to crimp tighter?
 
If they start out varying by .010, how do you know they grew. Did you measure the 6th round in the cylinder before firing the other 5?

Just asking. I loaded soft real soft nosed Rem JHP's the other day with a flat seater and I had the same problem with varying seating depth. I made a note to use the other seater stem which pushes on the ogive.

A little more crimp should solve the problem, either way, although .005 isn't bad, unless you keep doing it to that one round over and over. :)
 
It may be more of an indication that your present crimp is in the forward edge of the cannalure, allowing the bullets to slip forward a bit until they stop at the rear edge of the cannalure. The consistancy of the eight rounds suggests that's it and, if thats so, don't worry about it.

In fact, IMHO, .005" slip is meaningless no matter how it happens.

I suspect your WW bullets are about as good as any.
 
It may be more of an indication that your present crimp is in the forward edge of the cannalure
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Excellent point. The best crimp will have the front edge of the brass in the middle of the cannelure, where it is deepest.

As ranger335v pointed out, it is hard to get good consistent crimps with such a varied O.A.L.
 
I measured each of those rounds individually before and after loading them, so my measurements are accurate, regardless of the COAL variance among the rounds.

I too suspected that the bullet slips forward until the bottom of the cannelure comes in tighter contact with the crimped end of the case. I made sure that the crimp starts in the middle of the cannelure when I loaded the rounds. That is relatively easy to do, as the dimensions of the jacketed portion of the bullet are consistent, and only the exposed lead tips vary up to +/- 0.010 from bullet to bullet.

Based on the feedback I got from you, I consider the slip I measured as acceptable.

Thank you!
 
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