Why two cases of equal weight produce different pressure signs

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Newtosavage

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Saw something interesting at the range yesterday...

I've been loading 6.5 Grendel Lapua brass with 28 grains of IMR 8208 XBR and 123 SST's (0.5 grains under Hodgdon's max) after my ladder tests showed the first signs of pressure at 28.5 grains. So backed off to 28, life is good.

I happened to have 20 pieces of brass from some Hornady factory ammo, and I thought what the heck, I'll weigh them and check the volume against my Lapua brass, and load them to see what I get. So I did. To my surprise, the weight and volume was the same as my Lapua brass, to a tenth of a grain. So I started at 27 grains and immediately got flat, cratered primers. I was certainly not expecting that. I tossed the rest of that brass so I wouldn't get it mixed up with my Lapua brass, but it seemed surprising to me that just the brand of brass would make that much difference when the specs seemed the same.

Any thoughts? I'm just curious and trying to learn more about what creates pressure in handloads.

Thanks.
 
Just because two cases weigh the same, and the case volume appears the same, you don't really know for sure how that weight is distributed in the construction of the case. One could have thicker case walls, and thinner web than the other, and one may have thinner case walls and a thicker web. There could also be a difference in the alloy used to make the cases, or in the hardness of the brass between brands.

It's my belief that folks put too much emphasis on case weight. There are other variables that need to be taken into account, other than case weight alone.

Hope this helps.

Fred
 
I dislike Hornady factory brass. It always gives me early pressure signs.
Their bags of brass for reloading is decent.
 
Bfh_auto said:
I dislike Hornady factory brass. It always gives me early pressure signs.
Their bags of brass for reloading is decent.
Click to expand...
You know, now that you mention this I have to agree. I can recall early pressure signs with Hornady factory brass in both my 7mm-08 and 30-30. Something to that I think. In the future, I will avoid Hornady factory ammo if I plan to reload it. Although if I'm going to buy factory ammo, I usually get Federal brand.
 
Newtosavage said:
You know, now that you mention this I have to agree. I can recall early pressure signs with Hornady factory brass in both my 7mm-08 and 30-30. Something to that I think. In the future, I will avoid Hornady factory ammo if I plan to reload it. Although if I'm going to buy factory ammo, I usually get Federal brand.
Click to expand...
I usually go for Sellier&Bellot, Nosler, or Winchester.
 
Toprudder said:
Primers, by themselves, are not always good pressure indicators. I would really like to know if there was a difference in velocity.
Click to expand...
There wasn't. Same velocity, so the chrono didn't indicate a problem. But the primers sure did!
 
Newtosavage said:
There wasn't. Same velocity, so the chrono didn't indicate a problem. But the primers sure did!
Click to expand...

Headspace?? The only other time I've seen something like this was with brass that had the shoulder set back too far in FL sizing. Velocity was normal, but it sure as heck flattened the primers
 
Just mic'd the primer pockets and they are exactly the same.

Don't have a way to mic the flash holes but to my eyes, the Hornady look larger if anything.
 
Traditional pressure signs don't show up until you hit about 70,000 PSI. Which is about 20,000 PSI over what most 6.5 Grendel loads show as max. Just because you backed off 1/2 gr doesn't mean those loads aren't over pressure too.

There are so many variables that it is hard to say. I don't like to load without a chronograph. That way I know when I'm reaching speeds that indicate pressure. Most of the time book loads, even those near max don't give any trouble. But certain combinations of barrels, powder, bullets, primers, and cases will get you over pressure well below the max powder charges listed in the manuals.

I've only had that situation happen one time, but it was eye opening. I'd read that a certain powder was doing a good job with a certain cartridge and giving a bit more speed than the powder I was using so I bought some to try. Lots of positive comments from others who used it. At 59 gr everything was fine and the velocity was well under the max I was expecting. But at 60 gr of powder I suddenly got velocity 100 fps faster than it should have been at a max load of 63 gr. I never got any traditional pressure signs, but was obviously over loaded. While that combo worked for others, in their rifles, it was not safe in mine.

Going back to the original powder my chronograph showed exactly the same speed my loading manuals said it should be with no problems.
 
Its interesting reading, but doesnt apply to all cartridges.

Measuring the web thickness between the 2 brands, may show something? But with volume the same, would seem to mean very little.

Different Components=Different Pressure.
 
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Heck even the ejection rim cutout might be a smidge wider or deeper in one brand. This would allow for the water weight/volume in one brand to be different while both still have the same total weight. Also my understanding is that flash hole size has at least some effect on pressure as well. generally I do sort by headstamp in rifle brass and work up loads with each just for more accuracy potential. YMMV
 
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