Bullet Jump vs Pressure vs Velocity?

Victorian · Member Joined: Jul 07, 2014 Posts: 29 Location: Melbourne

I hope someone with a chonograph can help clear something up for me. I have a 7.62x51 ex-Israeli 98k Mauser (sporterised) that was showing pressure signs with some (not all) Winchester factory ammo (slightly stiff bolt lift, bright extractor mark on head). Doing some investigation it turns out this rifle has a short throat and factory ammo in/on the lands. (Headspace is also on the long side of 308W spec, but in spec)

So I have seated my factory ammo 100 thou deeper and I have been making my reloads about 100 thou off the lands and all is OK with compressed loads of Varget (AR2208) and 150gn Hornady SPBT.

I'm curious what this will this do to velocity (not that a fallow deer will care)? I read some places velocity can drop by 100fps or more. Other references show negligible drop in velocity. Not having a chronograph myself, when I get a chance I was thinking of comparing point of impacts at 100yds & 300yds to get an estimate of velocity variations.

Has anyone already tested velocity & seating depth?

I'm not too worried about extreme accuracy (getting about 1 moa @ 100yds - again a fallow deer won't care). Would seating say 50 thou off the lands (and not crunching my powder so much) get better velocity without too much more pressure? How close to the lands does pressure start increasing?

(attached pic is Win factory Power Point 150 gn)

tikkat3 · Super Member Joined: Jul 30, 2006 Posts: 800

I've just started playing around with one and have some averages for a 375H&H
3.5" 2856fps
3.6" 2885fps
3.7" 2887fps
Lands 3.8"

slimjim

www.huntingnut.com/ind...c&start=15

Victorian, I usually run my lead-core bullets about 0.015" off the lands and my monolithic copper bullets about 0.050" off the lands. At the bottom of the page in the link above you will find some testing I did on changes in seating depth and the impact to pressure and velocity. This was for my 6.8mm SPC II. Pressure did increase getting closer to the land for most of the bullets. I also found situations were pressure increased with a shorter COAL. And the bullets from GS Custom didn't change pressure with COAL. From this experience, I would postulate that there is a COAL where pressure is lowest and, if COAL is decreased or increased, pressure with increase.

Victorian · Member Joined: Jul 07, 2014 Posts: 29 Location: Melbourne

Thanks Slimjim,

With your 120g Hornady SST's what is the COAL touching the lands?

slimjim

Victorian, this is for my 6.8mm 18" ARP barrel with has less freebore than the standard SAAMI chamber.

120 Hornady SST__2.358”

95 Barnes TTSX___2.375”
110 Barnes TSX___2.438”
110 Hornady BTHP_2.335”
115 Nosler Custom_2.325”
120 Fusion MSR____2.475”
130 Nosler Partition_2.450"
130 Berger VLD_____2.475”
130 Hornady SST___2.485”
130 Nosler BT______2.505”
150 Nosler AB LR___2.560”