New Brass 50fps slower than fired brass?

[toasty]

So I have been developing a big game hunting load for my 6.5 creedmoor savage lightweight. All of my work has been with brass that has been fired at least 5x times. I found my load and then loaded up some new brass. I always expect a little bit of difference, and usually end up dropping .1 to .2 grains to compensate for the difference in velocity and poi in new brass vs once fired. Same dies and reloading procedures being used on both. Same environmental conditions and same magnetospeed chrono.

Load:
6.5mm Creedmoor
Hornady Brass
Fed LR Match
Varget - 39.1gr
123gr Hornady SST
2.780 COAL

For this load, I got 50 fps reduction in new brass vs my 5x+ fired brass. I will need to add about .5gr of powder to get back up to my load development velocity. Some things I noticed POI shifted by 1/2 moa to the left, same for elevation. 4 shot groups went from .40moa to .9moa. Neck tension was noticeably less with the new brass. These were shot on a clean bore. Most load development took place with 20-40 shots in the bore and was exposed to some HBN bullets down the tube. Are you guys seeing this much difference and what is an explanation that can cause the dramatic difference in velocity?

 

[Ned Ludd]

A certain amount of energy is required to expand virgin brass out to the chamber dimensions on the first firing. After that, the case is then resized minimally such that it fits more closely in the chamber, particularly at the shoulder. On the second firing, very little energy is required to expand the resized case, because it's much closer in dimension to the chamber. In my hands with both .223 and .308 loads, moving from virgin to fireformed brass is typically good for about 8-10 fps.

My guess is that you have something else going on in addition to the above process. Am I correct that this was comparing virgin brass from a new Lot to your previously 5X fireformed brass from a different Lot? If so, check the water volume of some fired cases (both new and the older stuff), perhaps the virgin brass in the new Lot simply has greater case volume to start than the previous Lot. If all other components (bullets, powder, primers) are from the exact same Lots, chances are good it will be some dimensional difference with the brass itself.

 

[T-shooter]

Had the same thing. I was dialing in a load. I retired the Hornady Match .308 brass I was using and loaded some new, same brand and part number (different lot), and prepped the same. I didn't have the chrono set up which would have probably told me something. With the exact same load, the group size went from .43 MOA to 1.1 MOA in similar conditions. I measured the volume of the cases and found the new brass averaged 4.1% larger and weighed 20 grains less. That explains it!

 

[toasty]

So I measured my new brass vs the old brass. Sure enough it has more volume. The new brass weighs 3% less than the old, but only measured 2% more volume. You guys think that is enough to drop it 50fps? How much affect do you think neck tension or a 30 vs 5 shot bore could have on the velocity?

 

[Twoboxer]

FWIW . . . ran Quickload using the data you provided, adding 26" barrel and using QL's 53.5gr water capacity. MV was 2856 fps. Increased case capacity by 2% to 54.6gr, and got 2832 fps.

 

[toasty]

FWIW . . . ran Quickload using the data you provided, adding 26" barrel and using QL's 53.5gr water capacity. MV was 2856 fps. Increased case capacity by 2% to 54.6gr, and got 2832 fps.

Thanks for the quick reply.

 

[ballisticdaddy]

So I measured my new brass vs the old brass. Sure enough it has more volume. The new brass weighs 3% less than the old, but only measured 2% more volume. You guys think that is enough to drop it 50fps? How much affect do you think neck tension or a 30 vs 5 shot bore could have on the velocity?

I stopped using Hornady brass even for hunting loads due to the inconsistency of the case volume, neck thickness and case weight. It is hard to get consistent loads while using inconsistent brass.

 

[dkhunt14]

So I measured my new brass vs the old brass. Sure enough it has more volume. The new brass weighs 3% less than the old, but only measured 2% more volume. You guys think that is enough to drop it 50fps? How much affect do you think neck tension or a 30 vs 5 shot bore could have on the velocity?

I believe your biggest factor is the energy required to form the brass. Then add in the capacity difference. Some of my guns are not up to speed at 5 shots especially if I cleaned them well. Matt

 

[Tiratore]

I've found measuring volume of water in the case with precision isn't as easy as weighing cases. It would be interesting to see if the calculated change in volume based on case weights produces a calculated change in velocity close to what Toasty measures. Brass density is about 7.62 times that of water (check my math). Knowing case weights, the difference in case volume in grains of water can be calculated.

 

[toasty]

I stopped using Hornady brass even for hunting loads due to the inconsistency of the case volume, neck thickness and case weight. It is hard to get consistent loads while using inconsistent brass.

This is my first time using hornady brass. I am not real impressed, a step down from the Lapua and Nosler brass that I usually use. I loaded up some +.3gr and +.5gr loads to chrono and check accuracy. I would bet that when I get back up to my original velocity, my groups should tighten back up. I will report the results.

 

[dkhunt14]

I loaded up some +.3gr and +.5gr loads to chrono and check accuracy. I would bet that when I get back up to my original velocity, my groups should tighten back up. I will report the results.

I would bet after they were formed you will gain a lot back. When I shot the 308 Baer, I had a fireform load tjat shot great. After formed I would need 1/2 grain less powder to match velocity of fireforming. Now this was with H4831 and a 240 Sierra. It was way bigger after formed. It take taper out the body and pushed the Weatherby radius shoulder out to 35 degrees. Matt

 

[Texas10]

The wild card I see in your results is the clean bore...in a factory barrel. That Savage barrel might prefer a bit of copper to smooth out the RR tracks left by the rifling process. You're results seem to confirm this.

New brass, clean bore, and maybe a hotter temperature the day of most recent test. That's a lot of changes to your loading recipe, IMOP.

 

[mikecr]

New brass never matters.
Form cases to what they will be, until then, you're nowhere but transition.

 

[Twoboxer]

I've found measuring volume of water in the case with precision isn't as easy as weighing cases. . . . Brass density is about 7.62 times that of water. Knowing case weights, the difference in case volume in grains of water can be calculated.

IMO, calculating that way is compromised by 2 assumptions . . . that you know the density of the actual brass in those specific cases, and that all of any weight difference is reflected in thicker case walls that decrease chamber volume.

 

[Tiratore]

IMO, calculating that way is compromised by 2 assumptions . . . that you know the density of the actual brass in those specific cases, and that all of any weight difference is reflected in thicker case walls that decrease chamber volume.

That's why it would be nice if someone with sufficient curiosity and ambition should run the numbers. Brass density variation is "probably" very small (it is, by internet sources, but who knows for sure), but the volume of the extraction groove should be evaluated. I expect ACTUAL case volume to be the important parameter here, but I've found case weight varies less than MEASURED case volume, suggesting volume measurement error may make weight a better figure of merit. But, as I said, it would be nice if someone ran the experiment ...

 

[T-shooter]

I found a few similar cases I weighed in the past. These are all Hornady .308 Match that have been neck turned fired, and full length resized. As a general rule, heavier cases have smaller volumes (at least in one brand/type) but nothing that's consistent. These were weighed dry with a spent primer (appx. 5 grains) and the water volume to fill the cases in cc's. I think that if you cannot or don't want to take the time to measure case volumes, weighing is worth while. Here, the lightest (by 20%) is only 7.4% larger than heaviest. Still enough to make a major difference in pressure.

158.1 grains 3.719 cc
169.4 3.601
169.5 3.614
172.3 3.596
174.4 3.511
174.5 3.492
181.4 3.525
189.6 3.469
190.1 3.464

 

[Twoboxer]

I'd guess many people quickly learned it's a PITA to measure water volume and have looked for a correlation between case weight and case volume, within and/or across headstamps.

My own day of trial occurred with 223 brass. It satisfied me that, like T-shooter's data shows, volume is generally inversely proportionate to weight . . . except when it isn't.

 

[mikecr]

Where most people fail in capacity measure, with this thread another basic example,, is that they don't take cases to fully formed/fully stable in dimension prior to concern or (worse) acting on testing results.

 

[toasty]

Where most people fail in capacity measure, with this thread another basic example,, is that they don't take cases to fully formed/fully stable in dimension prior to concern or (worse) acting on testing results.

Guilty as charged. I get what you're saying and I agree with it, my long range guns shoot new brass only to fireform it. This is for a 6lb mountain hunting rifle for my kids, 1/2 moa is plenty good for us as I won't let them shoot at a deer beyond 300 yards. I posted up becuase I have never seen such a large difference with new brass and trying to understand it. Probably volume, but maybe it is not volume. For factory hunting rifles with 20" ultra light contour barrels, new brass is OK to use IMO.

 

[mikecr]

Your latest lot new brass may have been factory sized a bit smaller. Then on firing, some pressure energy is used up in expansion to your chamber.
When you fire 6.5CM in your chamber, it becomes 6.5CM toasty Imp. It's special, as is the load you develop -for that.
After formed it might show no detriment to your favorite load (hopefully).