New 6BRA - Neck Turning prior to Fireforming

[scyzer]

Hi all.

I've got my first 6BRA with a .268 neck chamber. One quick question - this is one of my first rifles with a tight neck chamber where I need to turn virgin brass prior to the first firing.

I have turned a lot off brass down to 0.0110". I took the measurements with a Mitutoyo ball micrometer at different points on the case neck and across different cases and they seem to stay within a 0.0001" tolerance.

After fireforming and resizing, I'm seeing some cases with spots that are 0.0003-0.0004" thinner. Is that to be expected? Do case necks really become almost half a thou thinner in instances after the first firing? If so, should I expect less thinning in subsequent firings?

Also, does this mean that if I want to have my final prepped brass at 0.0110" thickness after fireforming and skimming, I shouldn't be turning down all the way to my target thickness on virgin brass?

Thanks!

 

[boltfluter]

If given the choice, I would fireforming before turning. Could not hurt anything and may actually help. When making Waldog cases I got my best results that way. Not an easy case to make uniformly imho.

Paul

 

[LitLBoy]

Partial cut, then fire form, and then final cut?

 

[scyzer]

Partial cut, then fire form, and then final cut?

Happy to do that going forward but just wanted to check if this was expected or if I may have introduced a variable somewhere.

 

[dnellans]

I turn before forming to .012. I’ve done close to 1000 of them and haven’t had the experience of them thinning further. Is the variance within a single case as you measure or some cases need a .001 smaller bushing to have the same sizing feel?

 

[Drolds72]

After you fire, the circumference increases. The same amount of material spread out over a bigger area would be thinner. Plausible?

 

[scyzer]

I turn before forming to .012. I’ve done close to 1000 of them and haven’t had the experience of them thinning further. Is the variance within a single case as you measure or some cases need a .001 smaller bushing to have the same sizing feel?

I've seen variances across a number of cases post firing, with the variations not being uniform around the case. Prior to fireforming, they were all spot on 0.011 regardless of where I measured.

After you fire, the circumference increases. The same amount of material spread out over a bigger area would be thinner. Plausible?

I was just surprised by the amount of thinning after a single firing - just wondering if anyone else has seen something similar and if the amount of thinning reduces in subsequent firings.

 

[Don Shelton]

I've seen variances across a number of cases post firing, with the variations not being uniform around the case. Prior to fireforming, they were all spot on 0.011 regardless of where I measured.



I was just surprised by the amount of thinning after a single firing - just wondering if anyone else has seen something similar and if the amount of thinning reduces in subsequent firings.

Following

 

[dnellans]

I've seen variances across a number of cases post firing, with the variations not being uniform around the case. Prior to fireforming, they were all spot on 0.011 regardless of where I measured.



I was just surprised by the amount of thinning after a single firing - just wondering if anyone else has seen something similar and if the amount of thinning reduces in subsequent firings.

Just to confirm - you're measuring thickness immediately after turning (241 mandrel used to turn) and then after sizing (presumably sized and expanded to 241 a well)? A fired neck (larger diam) nominally should have slightly thinner thickness, but .0003-.0004 seems like a lot. I've got some fresh turned brass in the shop and some fired a couple times. I'll measure and see what they look like.

 

[MikeMcCasland]

I cut mine straight out of the box. I actually prefer it that way. I haven't seen any issues.

.272 nk here too. I cut them for uniformity only.

 

[Drolds72]

Just to confirm - you're measuring thickness immediately after turning (241 mandrel used to turn) and then after sizing (presumably sized and expanded to 241 a well)? A fired neck (larger diam) nominally should have slightly thinner thickness, but .003-.004 seems like a lot. I've got some fresh turned brass downstairs and some fired a couple times. I'll measure and see what they look like.

He had 3-4 tenths. thinking if they were still warm after turning when he measured, that could be part of it too.

 

[dnellans]

If you’re using oil on the mandrel i’ve never had it get hot though to show up on measurements from start to end of a 100 case session.

 

[dnellans]

Here is some measured data from my bra for 3 spots on 5 cases each. All these 3 sets of brass were turned months apart from each other shooting for .0120-0121 as the target when adjusting the cutter and the 4th is unturned and fireformed. I know that .0120 doesn't always clean up 100% of the cases, but I'd guess it cleans up 90% of the cases 100% and the remaining 10% get a 90+% cleanup (just on visual swags). I do .0120 because its a lot easier to remember than .0116 which is the smallest I think I've ever measured. Because my brass hits the ground and can get stepped on, I don't want a super thin neck, just cleaned up.

I was happy to see that my .0120 target seems to hold pretty well over a bunch of firings and clearly halves (or better) the variance over unturned. I haven't remeasured anything after turning in a long time to see if it changes, so this was useful - given numbers below I'm probably going to just stick with what I'm doing.



I can't really explain why the freshly turned at .0120-.0121 target went up to .0122-0.0123 aside from the fact that after turning I run them through my normal bushing die to get them back to .002 neck tension rather than .001. Maybe that is changing the surface fininish on them just a tad so they measured differently or who knows... .0001-.0002 ain't much and if you can blow through that if you're not trying to be dilligent about how you're measuring with your tool! Its within the variance of all the other cases in the group measured, so I'm not sweating it, maybe they'll shrink .0001 on firing who knows

Hope the data helps you in some way.

 

[RegionRat]

@scyzer I‘m not a bench rester, but I’ll suggest you take some micrometer readings over the seated bullet to get a better picture of your neck thickness average and variance, regardless of the processing order.

When we are getting down into readings below 0.001”, things like the hardness, the modulus, the anvil shapes and Hertzian contact stress, and the surface roughness, can all have their effects on the uncertainty of your readings. Add in any variability in your dexterity and it isn’t difficult to get noise on the order of 0.0003”.

I still use a ball shaped anvil on a tube mic, but in the end I rely on the numbers from a regular micrometer over a seated bullet for a less technique sensitive measurement to avoid surface profile from clouding the issue.

 

[scyzer]

Much appreciated all. I'll double check some figures off loaded rounds but I take your point and won't get too hung up on the numbers.

The only real issue for me is that until I get my custom .260 neck sizing die, whilst 0.0003-0.0004" isn't very much, I'm on the limit of sizing with my current non bushing die (with a .263 neck).