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Old January 13, 2013, 11:12 AM   #7
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,825
Rodfac, yes, a nylon washer. I put one on a bracket that clamped onto my two rods that have two ball bearings (less than .0004" runout on their outside diameter) at the back for the case pressure ring to ride on. That bracket had a 1" OD, .39" ID nylon washer positioned such that the .308 Win. round was fairly straight into it when the case shoulder fit into the hole. Then pressed the back of the case so its shoulder pressed firmly into the washer and turned the round holding its back end down on the roller bearings.

The hole in the nylon washer was uniform enough that a GO headspace gauge in it showed less than .0001" runout on its shoulder's small diameter; good enough for my use. And that nylon washer didn't scratch brass case shoulders like the Neil Jones neck-shoulder bushing die did I used to simulate the chamber's shoulder.

Unclenick, I don't understand why many of Berger's bullets need a jump-start into the lands. All the 30 caliber Sierra's I've shot shoot more accurate when seated into the lands. Even testing Sierra's with runout less than .001" with different distances back from the lands when chambered, the further back they were, the less accurate they shot. Not a whole lot of accuracy improvement when jump was more than about .010" but from zero to .010" it was more noticable.

And minimal muzzle yaw and correspondingly slightly lower drag over the entire bullet's flight increases the average BC the bullet has. Which is why perfectly balanced bullets fired straight into the rifling without being further unbalanced by swaging down to the bore profile will have the least coning and drag, which, with the smallest spread in muzzle velocty means the tiniest groups.
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Last edited by Bart B.; January 13, 2013 at 02:05 PM.
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