Headspace question

[Nimbus73]

How much influence does headspace/shoulder bump have on velocity, sd/es and groups? I resized 26 pieces of brass yesterday for some confirmation loads. I was aiming for about a 1.6265" bump on my 243win. About half measured 1.6255 to 1.6265 but the rest varied all over as much as back to 1.622. I'm not sure why that happened. I normally have much better consistency when I size. For clarity, I'm sizing 15 rounds of 6x fired Hornady and 11 rounds of 1x fired brass. Some of the 11 rounds will be foulers, but 5 will be for a different charge weight test than the first 15 rounds. I don't anneal, so could this be signs brass hardening?

 

[KyleR1985]

are the 6x fired measuring different than the 1x fired?

 

[Nimbus73]

are the 6x fired measuring different than the 1x fired?

I'm at work and don't have my notes, but yeah they are a little different. I'd be lying if I quoted the averages, lol. Also, the 1x brass was fired from a previous barrel, so I expect some fire forming from those to the chamber of this barrel.

 

[KyleR1985]

If the 6x are are 1.625+, and the 1x are all 1.622+ you probably have your answer.

 

[Nimbus73]

I just finished loading 60 pieces of R-P brass that I have had for many years and 12 pieces of Winchester. I found that the aluminum lock ring on my Lee die had cracked at the tensioning screw. That may have been the issue. All of the brass had been fired from my old barrel and the shoulders measured about 1.624 before sizing. I bought several hundred rounds of factory 2nds that appear to be 100gr Pro Hunters. I was able to resize my brass to 1.6265 which is right on the money for what will give me about a 1.5 thou headspace for the chamber on my new barrel. I loaded 10 rounds each with near book max of H4350, H4831SC, N560, H1000 and Ramshot Grand for fire forming. I'm using a 2.244 ogive seating depth which gives a 2.648 COAL plus or minus a smidge. My gun likes the 100gr Sierra SBT with a 40.3gr dose of H4350 in a 20" barrel. I'm trying the different powders just to gauge what velocity they will give. Of these powders, the only one I've pushed past book max is the H4831SC up to 44.9gr which gave a 2825fps velocity with the SBT. The highest H4350 I used was 42.5gr at 2845. After some extensive research, I've determined a reasonable velocity for a 20" barrel is about 2850. I have realized achieving 2960-3000 fps is expecting too much with my short barrel. Comparing that to the 2690-2760 some commercial loads using a 100gr bullet is giving, I'm happy with the 2830 SBT loads that is giving me a .6 moa. When I stepped back to look at the real velocities that I get with store bought ammo I can't complain about my results. It will take a while to fire form all those loads, but I will get an idea of what the slower burning powders will deliver.

 

[Vern400]

I try to keep my brass and organized groups but sometimes it just gets mixed up. If I don't anneal I have trouble getting consistent shoulder bumps. And when you're measuring, don't bump the same case twice because it gets harder each time. Your final result won't be the same as another piece of rest that you just hit once. I've accepted a result of "everything closes easy, and not more than 0.003 clearance.
It's a lot better than just screwing the die down tight and cramming the shoulder wherever the die maker decided it should end up.
If you're jumping bullets it matters less. If you're shooting secant ogive bullets and running strangely close to the rifling it might matter more.

 

[eaglemountainman]

The 6x brass is more rigid/brittle and probably not springing back some, like the 1x are, after running through the sizer. For more consistency, sort brass by number of firings. For best consistency, anneal after every firing. I know, I know, it's an extra step...but you asked....

 

[wind gypsy]

Recently i was sizing 2x Lapua brass and then went to size a batch of 1x fired norma brass of same cartridge fired in same barrel. Without adjusting the sizing die the measured shoulder bump was a couple thou different if i recall correctly. So variance in brass ductility can easily cause differences in sizing.

 

[Vern400]

Recently i was sizing 2x Lapua brass and then went to size a batch of 1x fired norma brass of same cartridge fired in same barrel. Without adjusting the sizing die the measured shoulder bump was a couple thou different if i recall correctly. So variance in brass ductility can easily cause differences in sizing.

You're absolutely right. Between the hardness changes, and the wall thickness changes, it's near impossible to get precise shoulder bump.

I started annealing when some of my bullets went in really easy, and some of them we're much harder to seat. Same head stamp but from different lots.

 

[Tbieberle42]

You should have better results keeping all the brass together but I'd recommend trying to anneal some with a drill and socket. You may be surprised with the results. If your 6 firings in then I'd guess they are starting to work harden and hurt consistency.

 

[Nimbus73]

You should have better results keeping all the brass together but I'd recommend trying to anneal some with a drill and socket. You may be surprised with the results. If your 6 firings in then I'd guess they are starting to work harden and hurt consistency.

I do have a set of the bushings made by Little Crow Gun works, but have not really messed with them much. I do keep my brass sorted by head stamp, and number of firings. Most of my tests are done with boxes of 20, although I have a few boxes that aren't full. If I'm doing a confirmation test, I use a box that has one less firing than that I'm comparing against. I know that case hardening is a factor and since I don't anneal, at least for the foreseeable future, I am considerate of that. When I was prepping my R-P brass the other day so it can be fire formed, I realized my brass collection is too varied, so eventually I will reduce to just just one or two brands of brass. I'm also working on loads for multiple calibers I have so I have a lot of things I'm working on and my progress is slow between other obligations. For my 243, I am finalizing two different loads for FC brass that has multiple firings on them. I have one more bullet I want to develop in that gun. That's what I plan on using my 1x fired R-P brass for. I know that a load using X amount of powder my FC brass may not be ideal in my R-P or any other brass I may use, I'll have to back the charge off a little and work up to it, but it shouldn't be too far off. Being that my loads are primarily for hunting I'm tolerant of some inconsistencies. What I'm mainly trying to learn is what are normal inconsistencies and what are abnormal inconsistencies. I'm still developing my loading skills and methods and I learn something new everyday.