In another thread it was discussed that differences in case capacity between brands can lead to over pressures. Is there any correlation between case weight and capacity - i.e. the more a case weighs, the less capacity?

As part of my research, I weighed 5 cases of 7.62x39 from 5 different brands and took the average weight:

PMC - 116.24gr

FC - 117.82gr

Winchester - 122.5gr

Fiocchi - 119.24

S&B - 119.24

Could I assume that the Winchester has the least case capacity since it has the highest average weight? Also, I was surprised by the amount of difference in weight of the 5 cases. It seems all the brands had a pretty wide range with a couple with more than 6gr between the lightest and the heaviest case.

I wouldn’t know about the Winchester case having the least capacity but within a specific same brand, I have yet to measure a heavy case that didn’t have less case capacity than it’s lighter counterpart.

I don’t think it would be safe to assume anything when it comes to max charges and different brands of brass. A chronograph is your best friend.

I posted this before, but it relates to your question. It's fair to say that the heavier the case, the less volume. This is because the case measures the same externally, so any weight difference will mean the internal volume has to change. Heavier brass - less volume. Lighter brass - more volume.

Sometimes this difference is inconsequential. But it is not always so. As well, smaller cases will see faster, more aggressive swings in pressure.

Smaller cases like the 7.62x39mm or 223, are more sensitive to changes. These differences can take a safe load to unsafe, just by switching the brass you use. You will get a large change in pressure for not much of a velocity increase.

In 2008, I tested six different 7.62x39mm cases. Here are some of the results. There have always been differences in weight and volume with different lots made by the same manufacturer.
---

Empty Case Weight by Manufacturer - without primer

Igman 105.8 grains
Prvi Partizan 111.9 grains
Federal 118.0 grains
Winchester 122.1 grains
PMC 122.4 grains
Lapua 126.6 grains

Case Capacity – water weight in grains

Lapua -- 33.6 grains
Prvi Partizan -- 35.2 grains
Igman -- 36.0 grains
Federal -- 37.6 grains
PMC -- 37.6 grains
Winchester -- 37.9 grains

Pressure Differences Caused by Changing Case Brands

The powder charge, bullet and primer remained the same, but look what happened to the pressure when the case capacity was reduced. The 'Pressure by Case' goes from the most to the least internal volume. As case volume decreases, pressure increases.

These volume differences show why you should always start at the minimum load listed!!

The Load
Bullet - Lapua FMJ
Primer - Winchester Large Rifle
Powder - 28.5 grains of AA1680
Cartridge over all length of 2.200 inches

Maximum Safe Pressure - 51,488 PSI

Pressure by Case

Brand -- Pressure (PSI) -- Percentage of case filled
1. Winchester -- 50,264 -- 86
2. PMC -- 51,150 -- 87
3. Federal -- 51,150 -- 87
4. Igman -- 56,522 -- 91
5. Prvi Partizan -- 59,701 -- 93
6. Lapua -- 67,921-- 99

I would doubt there is much capacity difference in the cases you weighed, assuming they were the same trim length and all had primers in or all removed. In the most extreme case weight difference I have observed, there was a significant delta in capacity. That instance was 7mm Weatherby brass. Hornady weighed 243 gr and Weatherby weighed 212 gr. Hornady was less volume and produced pressure signs with less powder. I learned to carefully check case weight/capacity after that even between same brands of cases. Federal and Remington brass seems to be close in some cartridges, Winchester often lighter/more volume. Your experience may vary. Happy Trails

Thanks Steve - it's interesting that the Winchester and PMC cases were heavier than most of the rest, but had the largest measured case capacity...

Ya, how is that accomplished? Brass material make-up?

Water capacity of FF'd brass and COAL for your individual load will tell you more than will the weight of the brass. None of the above will tell you the whole story sans chrono.

There are a number of reasons why capacities vary, but they can change lot to lot. Sometimes, companies have their brass manufactured by someone else. There have been changes to ammunition/component specifications as well, although this last thing doesn't happen often.

Web and wall thickness also play a role. When they draw the cups while forming the cases, small variances will occur. A few grains isn't much. You would have to speak with an engineer to find out how much of a swing is brought about by 0.001 variance in wall or web thickness.

"measured case capacity" can be wildly innacurate, unless you're doing something to ensure the outside dimensions of each case are identical. It's important to think about the difference between a case sitting there on your bench, and the same case under pressure formed to the chamber walls when it fires. What you're measuring on the bench is not the same, and there can be significant capacity differences between otherwise identical cases.

Even if the cases were all fired in the same rifle (or sized in the same FL die), variations in brass hardness means variations in spring-back from the chamber or sizing die walls, which results in case capacity differences. In my experience, case weight is a much more accurate indicator of actual capacity.

There can be some error in sorting by case weight, but the biggest uncontrolled variable there is the dimensions of the extractor groove. Every other area of the case is enclosed by the chamber and bolt, and additional brass in any of those areas directly correlates to case capacity during the firing cycle.

I agree with all of that.

There are some differences in composition as between different makers and perhaps lots of brass. See for example https://www.accurateshooter.com/technical-articles/x-ray-spectrometry-of-cartridge-brass/ . However the difference in SG across this fairly narrow band of composition is not very much, perhaps +/- about 1 %.

The extractor groove dimensions, and things like the extent of bevelling of the case head, and perhaps also case mouths and primer pockets, will be more significant variables in the weight vs capacity i think. If you look at different brands of brass there can be quite wide variation in how much metal is removed in cutting the extractor groove and bevelling the case head. If it wasn't for these, you'd expect that for a given external size the weight of the case would be a very good proxy for internal volume.

Can't find the article but believe it was also from accurate shooter, they weighed several brands of cases and even though the weights varied widely the capacity range differences was fairly small. It was even less than what Steve Redgewell found, I doubt this is true for all cartridges but it was for the 243 cases that were weighed in their sample. MD has explained his method or water capacity using a seated bullet several times, I think this is the best way after all bullet prep has been done and after two firings.