nucking up 243 to 260 Rem

[NZVarminter]

I've been necking up Lapua 243 brass to 260 Rem, but have not been getting consistent accuracy.

one week I'll shoot a 0.3" group, the next week the same load goes over an inch.

This has happened with 95 VM's, 129 Hornardy SP, 107 Sierra's. A friend has suggested there might be an issue with necking up the 243 brass and the dreaded "donut" forming at the base of the neck. well I've checked and there definitely is a thickening at the base of the neck. i can slid a projectile in a fired case and it stops dead at the point.

I'm thinking this might be contributing to inconsistent neck tension which which explain my inconsistent accuracy. I suspect my best groups were probably with virgin brass, and accuracy has gone down hill since so that supports the theory that the donut is affecting accuracy.

So, I've got some factory Rem brass to try, but would like to stick with Lapua as its no dearer than Rem brass over here (NZ). I assume if I neck down 308 brass, that the donut is not an issue as the new neck/shoulder will be formed in the neck area and not the thicker shoulder material.

Cheers

Grant

 

[Cruizin]

I would imagine both necking up and necking down would cause material thickness changes in the necks, but I would think necking up would be less of an issue with donuts.

Are you using busing dies to reload? If so this could be the cause of the donut.

I would double check all your other reloading techniques before getting into turning the necks.