Resizing Brass?

[ABL_ZA]

I am going to be resizing fired brass for the first time, as I have only ever used virgin brass before.

Rifle: Tikka T3X 6.5 Creedmoor
Virgin brass at 1.910 with a seated CBTO of 2.19

I have always used a CBTO measurement as my final measurement to maintain consistency. After firing, I anneal and clean the brass. I use a mandrel for the necks and then trim the case to 1.910

My concern now is that the brass size will still be different to that of the virgin brass.

Questions:

1. My fired brass all exhibits different measurements using my Area 419 headspace Gauge. I suspect different brass expands and springs back at different rates. What would an acceptable tolerance be? If we uniformly resize less 0.002 then will this not impact the CBTO length that I use as my go to seating measurement?
   • I expect If dies and setup consistently bump all brass back the same amount (e.g., 0.002" less than fired measurement), the CBTO variation will be minimized. However, since brass length and case volume change with firing, some variations in CBTO may still occur.
2. How much does this affect the load over time if we resize less 0.002 every time? Should we limit resizing + annealign to say 8 times for max safety/accuracy?
   • I expect case wall thickness may still change over multiple firings. If neck tension varies, bullet release pressure will vary, which affects velocity and pressure. I hope the change over the lifetime of the brass should not extend beyond anything that should significantly affect accuracy, enough to warrant new load dev etc.
3. If I load the fired brass to the same CBTO, will the load react in the same way as with the virgin brass? Should I weigh fired vs. virgin brass to check for volume differences and scrap the outliers?
   • Fired brass may have different neck tension so I expect case volume will change leading to minor pressure and velocity changes as wall thickness changes over multiple firings.

 

[spife7980]

Case base to ogive doesn’t have anything to do with sizing brass, ogive is in the bullet. Set the die so that your shoulders are .002 less than fired, good enough.

 

[ABL_ZA]

Case bast to ogive doesn’t have anything to do with sizing brass, ogive is in the bullet. Set the die so that your shoulders are .002 less than fired, good enough.

Would inconsistent resizing or case headspace growth not alter CBTO consistency?

• If your brass is stretched unevenly (inconsistent resizing), bullets may seat at slightly different depths because shoulder bump inconsistency affects where the bullet sits in relation to the lands.
• If you trim cases unevenly, the seater die may press bullets slightly differently, affecting CBTO.
• If neck tension varies, seating force changes, impacting how deep the bullet sits even when targeting the same CBTO.

 

[spife7980]

Yeah technically cbto relative to the chamber but you can worry about that later and find out it’s not really gonna matter.

If the cases aren’t all trimmed the same then it won’t really matter. You’ll find that out as well. Just don’t let them get long enough to pinch a bullet at the end of the neck.

Neck tension is mainly going to be driven by interference fit and neck surface, you can change the neck bushing but other than that it is what it is for now.

 

[ShtrRdy]

I admit, I didn't try to comprehend your verbose questions.

Fired and/or resized brass will have different measurements than new brass. The first firing of new brass will "fireform" the brass to the chamber it was fired in. That's why it's a good idea to use the same group of brass for a particular rifle/chamber. Then when you resize brass it will have dimensions that are different that the new brass or the resized brass.

The dimensions of the resized brass are the ones we're interested in.

 

[Old Man with Gun]

Just shoot it, unless you see a difference there is no reason to overthink it.

 

[straightshooter1]

I am going to be resizing fired brass for the first time, as I have only ever used virgin brass before.

Rifle: Tikka T3X 6.5 Creedmoor
Virgin brass at 1.910 with a seated CBTO of 2.19

I have always used a CBTO measurement as my final measurement to maintain consistency. After firing, I anneal and clean the brass. I use a mandrel for the necks and then trim the case to 1.910

My concern now is that the brass size will still be different to that of the virgin brass.

Questions:

1. My fired brass all exhibits different measurements using my Area 419 headspace Gauge. I suspect different brass expands and springs back at different rates. What would an acceptable tolerance be? If we uniformly resize less 0.002 then will this not impact the CBTO length that I use as my go to seating measurement?
   • I expect If dies and setup consistently bump all brass back the same amount (e.g., 0.002" less than fired measurement), the CBTO variation will be minimized. However, since brass length and case volume change with firing, some variations in CBTO may still occur.

If you measure virgin brass to start with you'll find "different measurements" to start with. Then, as you surmise, different brass indeed can spring back differently. Some brands of brass are produced more uniform than others. It's doesn't matter if you resize to .002 or .001, though that is only one factor of several that will affect your seating measurement.

CBTO variation will me minimized by your control over the interference between the surface of the bullet and the interior neck surface. Here's an article that explains that in detail: https://bulletin.accurateshooter.co...bullets-neck-bushing-size-is-only-one-factor/

Note: after 2 or 3 firings, your cases should be fully fire formed, which will help with getting consistent sizing measurements.

2. How much does this affect the load over time if we resize less 0.002 every time? Should we limit resizing + annealign to say 8 times for max safety/accuracy?
I expect case wall thickness may still change over multiple firings. If neck tension varies, bullet release pressure will vary, which affects velocity and pressure. I hope the change over the lifetime of the brass should not extend beyond anything that should significantly affect accuracy, enough to warrant new load dev etc.

Every time you fire a case the movement from firing and the resizing process makes the brass a little harder. . . each time. When it gets harder each time the springback changes each time along with the interference, which can change your "precision" results on target (note the difference between accuracy and precision). How much it affects that will have to do with how much movement is going on with the brass for each firing and resizing cycle. Reloaders with custom chambers, reamers and dies can really minimize a lot of these issues and get good results over many firings.

Proper annealing after every firing is not going to be a problem for safety, accuracy or precision.

Case wall thickness does change over time with many firings, but the change is very very small. . . . pretty much just nothing to be an issue at all. Case hardening is the biggest issue as the brass movement of case goes through the cycle of expansion and contraction.

3. If I load the fired brass to the same CBTO, will the load react in the same way as with the virgin brass? Should I weigh fired vs. virgin brass to check for volume differences and scrap the outliers?

• Fired brass may have different neck tension so I expect case volume will change leading to minor pressure and velocity changes as wall thickness changes over multiple firings.

Basically, no. . . loading fired brass to the same CBTO will not necessarily react the same as the virgin brass, unless . . . . all other factors are the same to produce the same interference.

Because there is a correlations between case weight and case volume, weighing the case and culling the outliers does help with getting consistent results on target and with the chrono data (like ES's and SD's). I've done this and it works nicely. In addition, I've grouped weights that are close together that produce really good results. Whether you weigh the cases when they're virgin or after firing, you're going to find the same variance. Just make sure when you weigh fired cases, the primers are removed as their are variations there too.

Being a skeptic, I wasn't convinced that sort weighing primers made any difference until I tried it. I found it does make a difference in consistency (as measured in ES's and SD's) when culling out the outliers.

The case volume that's important is the fired case volume, not the virgin or sized case volume. This has to do with the maximum expansion the case does while in the chamber during the ignition prosses where it's restricted the the chamber walls.

Again. . . there's not enough change in case wall thickness to be of any concern (it's really really small amount for many firings). The only change in case wall thickness that tends to be of concern is where the donut forms at and just above the case shoulder-neck junction where it may make contact with the bullet's bearing surface depending on how far down the bullet is seated.