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View Full Version : Velocity Variations - Help please



thekubiaks
09-28-2008, 07:17 PM
I am having some velocity variations that I can't figure out. My 6.5 x 284 LG has a .290 neck. I neck turn the Lapua Brass to a wall thickness of .012" This gives me a .288" round that I size to .286 to give me .002 of bullet tension. When I shoot my Sierra 1742's (142gr), I am getting single digit veolcity variations as long as the round is just off of the lands or just barely touching. As I start to push the bullet further into the lands, I am starting to get 20 - 30 fps velocity variations.

I know that pressure is going up dramatically once the bullet is in the lands but why the jump in velocity variations?? Accuracy is getting better as I go about .010 - .020" into the lands but, velocity starts moving around which isn't good for 1000yd shooting.

Any thoughts?? Thanks for the replies.

John Kielly
09-28-2008, 07:37 PM
I found a similar result when I was loading molied 210 .308 Bergers, I sweated & worried about it until one day I accidentally set my loading die one full turn too far & gave the bullets jump. Damned if it didn't smooth out the velocity variation and give me good long range accuracy as well.

I later read that Boots Obermeyer contends that it is necessary to jump molied projectiles .015-.060" for them to achieve the necessary consistency of start up. I understand that his theory is that by jamming them, you have vatiation in what amount of moly is removed by the lands & this screws up consistency of friction. Who knows?

Anyway, as long as my throats are cut .0005" over projectile diameter & while I can assemble my loads srtaight, it's working for me.

JerrySharrett
09-29-2008, 05:18 AM
Is this just happening with one powder or one lot of one powder. Or is it happening with different powders?