Headspace variance off shoulder bump

[07jkjeeper]

I've got some once fired brass that working with on my 300WM. Same lot of brass. All brass was annealed prior to resizing. My new brass measured 2.249" and the fired brass measured at 2.2705". .0215" expansion! Both messurents were taken from base to datum. I'm learning that maybe new brass should be measured from the belt on 300WM then fired brass from face to datum.

My issue is two fold. When using the body die I am getting very inconsistent sizing as spread out as much as .004", most within .001". The other issue is that I managed to bump the shoulder back .007" rather than the .004" that I intended. I use imperial case lube with a Redding Body Die and comp case holders. Bonehead move, I messed up by using the case holders out of order from large to small getting max bump on my first push. The extra spacing isnt ideal for fire formed brass but I would think it should be fine shooting since it's a heck of a lot better fitting than the new brass that stretched .0215" on the first firing.

How much change in poi and pressure should I expect with the shoulder bump so far from the fired state (.007") and how much does .001-.004" variance in resizing (shoulder bump) affect things. I'm hoping I dont have to fire form all of this back out or worse scrap the brass.

 

[Cloudrepair]

I've got some once fired brass that working with on my 300WM. Same lot of brass. All brass was annealed prior to resizing. My new brass measured 2.249" and the fired brass measured at 2.2705". .0215" expansion! Both messurents were taken from base to datum. I'm learning that maybe new brass should be measured from the belt on 300WM then fired brass from face to datum.

My issue is two fold. When using the body die I am getting very inconsistent sizing as spread out as much as .004", most within .001". The other issue is that I managed to bump the shoulder back .007" rather than the .004" that I intended. I use imperial case lube with a Redding Body Die and comp case holders. Bonehead move, I messed up by using the case holders out of order from large to small getting max bump on my first push. The extra spacing isnt ideal for fire formed brass but I would think it should be fine shooting since it's a heck of a lot better fitting than the new brass that stretched .0215" on the first firing.

How much change in poi and pressure should I expect with the shoulder bump so far from the fired state (.007") and how much does .001-.004" variance in resizing (shoulder bump) affect things. I'm hoping I dont have to fire form all of this back out or worse scrap the brass.

This is why I like short mags no belts plus a little easier on the shoulder.
After fire forming I would try to keep shoulder bump to a .002 range. As you probably already know the win mag is head spaced off the belt.
I just made some tools today for a top tier brass maker and the tolerances are tight for the tooling but on a belted mag I'm not sure why it would seem acceptable for that much of a head to shoulder fit but I just make the tools.

 

[07jkjeeper]

Yeah I agree. I bumped back .002 but still had a tight bolt so went .004 and it was nice. Then I used the wrong shell holder and pushed the shoulder back to .007. Total mess up on my part. I'm not sure why I was getting the inconsistencies in the bump. The die had been cleaned and lube was applied to each case. I'm almost wondering if I used too much lube, no damage or dents to the shoulders.

 

[rsmithsr50]

real NEW brass is typically undersized...it always fits.
the once fired is what ever the other chamber was.
nothing to see here, move along

 

[FeMan]

The difference in case volume and resulting pressure difference between a 0.004" bump and a 0.007" bump should be almost nothing IME. Brass has a "memory" and tends to want to return towards where it came from. With shoulder growth like you mention, I am not surprised you have that much variability in shoulder bump on the first reload. I would expect them to normalize over the fist few reloads.

 

[Steve Ladino]

I've got some once fired brass that working with on my 300WM. Same lot of brass. All brass was annealed prior to resizing. My new brass measured 2.249" and the fired brass measured at 2.2705". .0215" expansion! Both messurents were taken from base to datum. I'm learning that maybe new brass should be measured from the belt on 300WM then fired brass from face to datum.

My issue is two fold. When using the body die I am getting very inconsistent sizing as spread out as much as .004", most within .001". The other issue is that I managed to bump the shoulder back .007" rather than the .004" that I intended. I use imperial case lube with a Redding Body Die and comp case holders. Bonehead move, I messed up by using the case holders out of order from large to small getting max bump on my first push. The extra spacing isnt ideal for fire formed brass but I would think it should be fine shooting since it's a heck of a lot better fitting than the new brass that stretched .0215" on the first firing.

How much change in poi and pressure should I expect with the shoulder bump so far from the fired state (.007") and how much does .001-.004" variance in resizing (shoulder bump) affect things. I'm hoping I dont have to fire form all of this back out or worse scrap the brass.

Getting your tooling to resize the web area of the case and not bump the shoulder on fire formed cases is key to case life and accuracy. In the worst case deal with a little more shoulder clearance thereby allowing web sizing and easy chambering or try another resizing die . My Lee FL die works for my 300 WM. Ruger LRT.

 

[Zero333]

If you're using these loads for hunting, you have nothing to worry about. Even if you plan on shooting out to 600 yards.

If you're trying to shoot the smallest group in the history of the universe, you might wanna fireform them again.

 

[Hondo64d]

Yeah I agree. I bumped back .002 but still had a tight bolt so went .004 and it was nice. Then I used the wrong shell holder and pushed the shoulder back to .007. Total mess up on my part. I'm not sure why I was getting the inconsistencies in the bump. The die had been cleaned and lube was applied to each case. I'm almost wondering if I used too much lube, no damage or dents to the shoulders.

If you’re bumping the shoulder back .002 and still have have tight chambering, your die may not be adequately sizing the area just forward of the belt. Turning the die down some more might take care of that but it also pushes the shoulder back further which is also not desirable. Might investigate the Willis collet die.

John

 

[searcher]

The pressure differential on the .002" shoulder bump should be minimal, but most importantly will impact each case the same, resulting in likely little to no group change. The .007" shoulder bump on the other hand creates a much higher probability of group dispersion. If using hard cup primers, you may encounter erratic primer ignition. As Hondo64d said, you are not likely sizing all the way down if bumping .002" and still having bolt resistance on closing. Other possibility, though less likely, is that your brass is in need of annealing badly - as it is mostly springing back, perhaps combined with a slight error in the measuring tool, or a primer that is not entirely seated or the overall brass length has exceeded maximum length. In that scenario- it required you to bump the shoulder to shorten the length - whereas you are thinking it is just a shoulder length problem.