Copyright 2002 – Stephen Redgwell
As a follow up to the 308 Winchester & 7.62x51mm article, I sat down with my digital scale and did what is done at every Weight Watcher’s meeting – I weighed things. Like Weight Watchers, I wanted to get a “Before & After” picture. I wanted to test and record three things:
– the weight difference between military and commercial cases.
– the volume differences between military and commercial cases.
– the volume differences before and after firing
CONTROL STATEMENT: I performed each test on military and commercial 303 British brass. I repeated the tests using 308 Winchester and 7.62x51mm NATO brass.
I always do this when working up new loads with untested powders or new cases. Oddly, few reloaders actually do these simple checks. It will help explain differences in velocity and performance.
A COUPLE OF THINGS TO KEEP IN MIND
1. Brass weight varies between manufacturers. It can also vary between different lots (production runs) from the same manufacturer.
2. The amount of change in volume between fired and unfired cases will depend on the rifle’s chamber. Is your chamber large, medium or small?
TEST ONE: Weight differences between military and commercial cases
CASE WEIGHTS – Empty (Average of 10)
177 gr IVI* (military)
165 gr Remington (commercial)
12 gr or 6.8% heavier
194 gr IVI* (military)
179 gr Lapua (commercial)
15 gr or 7.7% heavier
With both cartridges, the military brass weighs more than its civilian counterpart.
From this, we can conclude two things:
1. Because commercial and military cases share the same external (outside) case dimensions, but the military cases weigh more, then there must be more brass.
2. Because there is more brass occupying the same space, the volume will be less.
This is why you must never use reloading data from any book without first checking to see which type of case was used to develop the load. With my manuals, only Hodgdon specifies loads with commercial AND military cases. All the others use commercial cases only.
TEST TWO: Volume differences between military and commercial cases
How do we know for sure that there is less volume in a military case? That’s easy, we can use weight again, but this time we’ll add some water. Here is the procedure.
1. Take a fired military case with the primer still in it and weigh it on your scale. You should do this with about 10 cases and take an average. I will work it though with you.
Result – 177 grains – this is the empty case weight
2. Now take the same case and fill it with water. Fill it right to the top of the neck so that the water is dished slightly inwards (concavely shaped).
3. Weigh the full case. If you have a beam type scale, you can lay the case on its side if you’re careful. The water won’t come out.
Result – 233 grains – this is the full case weight
4. Subtract the empty case weight from the full case weight and the result will be the case capacity in grains of water.
233 grains MINUS 177 grains = 56 grains
That was easy! Repeat the same steps using commercial cases.
Now that we know the case capacity in grains of water for both the military and commercial cases, we can compare the two and see the difference. Commercial cases have a higher internal volume.
CASE VOLUME – Water Weight in Grains
58.8 gr Remington (commercial)
56.0 gr IVI (military)
-2.8 gr or 4.8% difference
308 Winchester/7.62x51mm NATO
55.7 gr – Lapua (Commercial)
53.0 gr – IVI (military)
-2.7 gr or 4.8% difference
Okay, that’s great. We can see the difference between military and commercial cases. There is more volume in commercially made cases.
This last part is for people that neck size their cases. If you full length resize, this doesn’t apply to you, but makes good reading.
TEST THREE: Volume differences before and after firing
Does the internal volume increase when we fire a new case and compare it to one that is once fired?
303 British – Commercial Cases
58.8 gr Remington (fired)
57.3 gr Remington (unfired)
1.5 gr difference or 2.6% more volume after firing
303 British – Military Cases
56.0 gr IVI (fired)
55.1 gr IVI (unfired)
.9 gr difference or 1.6% more volume after firing
308 Winchester – Commercial Case
55.7 gr – Lapua (fired)
54.5 gr – Lapua (unfired)
1.2 gr difference or 2.2% more volume after firing
7.62x51mm – Military Case
53.0 gr – IVI (fired)
52.4 gr – IVI (unfired)
.6 gr difference or 1.1% more volume after firing
Velocity can drop between the first time you use a new, unfired case and when you reload it using a neck resizing die.
SOME FINAL THOUGHTS
How much extra internal case volume is created upon firing will depend on how long your chamber measures and the thickness of the brass itself.
If you want to find out about the size of your rifle’s chamber or differences between different case manufacturers, try loading 10 new cases and shoot each over a chronograph. Record every shot and average the 10. Next, reload those same cases – neck resizing only – using the same powder weight, bullet and primer and repeat the test.
For fun, find out how much powder is required to restore the original velocity. The amount of powder will increase minimally, but the powder weight will increase!
For increased precision, start your work ups with once fired cases. Fireformed cases exhibit true internal volume, align better to the bore because of their closer internal fit and are generally more accurate than full length resized or new commercial cartridges.