Saturday, June 30, 2018

Case forming, The Shrinkage

Not much, but some.

Helping with the .310 Cadet loads, few days back modified a bunch of cases, then were able to fire thirty of them with a basic load.  It's not uncommon for cases, when fireformed in the chamber, to lose a bit of length.  This time I measured all the cases after trimming; a couple were a few thousandths shorter, but the rest all came out a 1.139".

The load was 7.0 grains 2400, a Remington Small Rifle primer, and the RCBS 120-grain bullet.  Thirty were fired, and twenty-five measured 1.130-1.131"; four were 1.128", and one was 1.125".

I suspect the five shorter may have been a touch shorter originally(measuring error on my part), so if you discount those these shrank 0.008" in length. 

You may ask how I came up with 1.139" for starting length: previous experiment showed that if you got much over 1.140('much' in this case meaning .003-.005) the bullet was being pushed into the rifling in order to close the action.  Which I don't like as a general thing.  So on 'if it's all consistent it's good' theory a case a few thousandths less than the useable max makes for easier loading, no worries about pressure spikes from having the bullet engaging the rifling, and, facing facts, this isn't a super-precise match rifle where that few thousandths is going to make a big difference in accuracy.

Something else I need to try: so far the bullets used were cast of 15-1 lead-tin alloy, I want to cast some of mongrel alloy and see how they work.  Since this bullet fits the bore nicely, I'm hoping for good results.

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