Neck Tension Changing Over Time

JD Tactical · Jun 5, 2020

Hey guys, new member here but not at all new to the sport. I thought I would share some interesting info that I learned in the last year. Apologies if this has been discussed before.

While working up a new load last year for my 300 RUM I noticed that my "fresh" loads (loaded within a day or two of shooting) had a considerably higher MV than loads that had sat for about two weeks or so. I am full length sizing all my brass the same way with a redding bushing die and the expander mandrel installed. All loads were the exact same specs but the fresh loads were around 3135 FPS with an ES of around 20 FPS and the 'seasoned' loads shoot around 3075 FPS with the same ES.

I know that a lot of benchrest shooters load and shoot immediately, even loading at their vehicle before the match. As a hunter I can't really do this so now I prefer to 'season' my loads for a couple weeks before shooting.

To back this up, a buddy of mine I shoot with noticed the exact same thing on his 6.5 CM while we were shooting together.

Simply put, I believe brass will 'stress relieve' over time which results in slightly less neck tension. This, I assume is causing this to happen.

LiveLife · Jun 5, 2020

Welcome to THR.

JD Tactical said:
All loads were the exact same specs but the fresh loads were around 3135 FPS ... and the 'seasoned' loads shoot around 3075 FPS

... I believe brass will 'stress relieve' over time which results in slightly less neck tension. This, I assume is causing this to happen.

Were the ambient temperature (specifically, powder charge temperature) the same as powder temperature can affect velocities?

To give validity to your claim of neck tension being affected by "brass seasoning", you would need to keep the shooting condition variables the same.

FROGO207 · Jun 6, 2020

My thoughts on the subject are that it is the VOC's (acetone etc. you smell) when you open a canister of propellant that are off gassing then escaping over time. The reduced amounts within the round reduce velocity.

If you wanted to test this theory just seal primer and bullet when loading, wait and shoot. If you do please share the results.

jmorris · Jun 6, 2020

14 days isn’t much time relative to the life of a brass case. If 14 days alone caused a change of -60 FPS, much less just in the case alone. I would have been DQ’ed from a lot of matches over the years.

If you want to continue your neck tension tests though I would seat out further, to the point the bullet is seated deeper into the case as the bolt is closed. If the velocity is still dropping by 60 fps with those rounds, something else is going on as once the bullet is held by the barrel, neck tension is much less important.

Be sure to reduce your loads accordingly.

Welcome aboard.

Hooda Thunkit · Jun 6, 2020

Another way to test if it is indeed neck tension difference; crimp the rounds.

If the rounds are all crimped to the same degree, the neck tension is less likely to affect anything.

TBH, if neck tension was the cause, I would think you would see the difference in the Extreme Spread, more so than velocity.

murf · Jun 6, 2020

welcome jd tactical.

rcmodel suggested you bullet and case are probably cold welded together. cold welding of similar metals can happen over time. i think there are some long distance shooters that "final seat" their bullets right before they shoot so this phenomenon won't happen.

you can use the search function to find the prior discussion.

luck,

murf

Neck Tension Changing Over Time

JD Tactical

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LiveLife

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FROGO207

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jmorris

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Hooda Thunkit

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murf

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