Home Forums Hunting & Shooting Ask The Gunwriters Case weight vs Capacity

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.
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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