I remember seeing some time back some rubber plugs to cap off the flash hole to add water to get a measurement of the case capacity but I now forgot where I saw them or who was making/ selling them. Do yall just make up something similar? How to you damn up the flash hole to add your water? Just looking for ideas.

I just put a used primer back in.

Wax from a candle or other source. When done, just heat the case.

I guess it’s OK to do that, but it seems like a waste of time to me. If you are looking for case volume consistency, it is far simpler to just weight match cases.

What about the water tension at the top of the case? That will throw off the weight per each different case.
I agree with simply weghing each one and get a match set that way.

21st Century sells plugs.

I use bits of clay.

I found using water to measure case capacity useful when loading cases with water.

God help me if I ever get to that point.

I used to sort my military match brass by weight when loading for my M1A super match and for my .308 precision rifles. I did the same with 173 gr FMJBT projectiles from DCM, as by that time the Lake City tooling that made them was worn and bu,let weights varied a lot.

I found that sizing and trimming the cases to the same OAL before weighing gave results that were just as consistent as weighing by water volume, with a lot less bother.

——

I did however revert back to water volume measurement during the first Great Component Shortage when I was fire forming .375 Win brass from new .30-30 brass.

I was curious to see how much thicker the case walls were in Winchester .375 Win brass compared to .375 Win brass formed from Winchester .30-30 brass.

I found that on average the .375 Win brass held slightly less water than .375 Win cases formed from .30-30 brass. However I also found about 50% of the cases over lapped between the two with about half of the .375 Win cases having water volumes that fell within the range of converted .30-30 cases and vice versa.

Once I’d measured water volumes for 100 of each, I repeated it with just case weight sorting and found the results were essentially identical.

It mattered in that situation as .30-30 runs at a maximum average pressure of 42,000 psi while .375 Win runs at 55,000 psi. By excluding the highest half of the water volume cases / lowest half of the case weight cases for the .375 Win cases formed from .30-30 brass I could ensure the case wall thickness was comparable to the thinnest of the factory .375 Win cases. Those cases were used for max pressure .375 Win loads while the lighter weight/ higher water volume cases were used with lower pressure loads not exceeding 42,000 psi.

But again, once I confirmed the sorting was essentially identical with either method, I just used case weight within the over lapping range of .375 Win as the standard method for sorting the entire batch of fire formed cases.

——

As for plugging the primer pocket I just seated spent primers into the pocket upside down. It’s easily removed by just running the case through a decapping die when you are done.

Water tension is constant no matter the size of the container.

I would think weighing would be less consistent due to brass density variations between manufacturers brass. It probably insignificant but would use volume over weight. Volume is absolute.

Surface tension is consistent provided the case is clean and the water is the same.

- Weigh the case;
- Fill the case;
- card it off; and
- re-weigh it to get the weight of the water.

——

There can be weight differences even in ordinance brass, but if you are using new brass from the same lot, differences should be minimal.

It’s important to understand that even brass with the same headstamp can vary a lot. For example Winchester no longer makes its own brass but instead contracts brass from different companies. That means Winchester brass you buy at one point in time can be significantly different than Winchester brass you buy at some other point in time, even though it’s the same cartridge.

The most egregious example will be an old LC Match case (Lake City ammunition plant) in 7.62x51 NATO and a new LC Match case in 7.62x51 NATO made under the Federal contract for rounds like the current M118 LR and the comparable USN Mk 316 Mod 1 and Mod 2 rounds. The new rounds are all made using Federal’s Gold Metal Match Case which is the same case used in their civilian .308 Gold Medal Match ammo. Since it is never intended to be fired in a machine gun with questionable headspace like the M60, it isn’t significantly heavier and thicker walled like the older 7.62x51 military and military match brass.

In short, the days when I could just mix and match all my 7.72x51 military Match brass are gone.

——

On the other hand, water volume isn’t perfect either. You want to use brass that is consistent in external dimension - either resized in the same small base sizing die, or fired in the same chamber at the same average pressure. But even then there can be differences in how much the brass snaps back that can affect the internal volume of the case.

So in the end, you control as many variables as you can, but there will still be variation that affects the validity of measurement, whether it’s weight or water volume.

I use a BB size ball of plumber's putty in the primer pocket & then press it in until it starts to fill in the flash hole.

.

__________________
Waiting for the break of day

The upside down primer is probably the easiest way to plug up the flash hole and like said above, it's easy to remove also.

Don't over think this, do it the easy way and it will be accurate too.

__________________
Freedom is never free!!
SWCA #3437

How to plug the flash hole? I have cases mocked up like this for various tasks. I place a piece of plastic wrap over a spent primer then seat it. The primer acts like a cookie cutter and makes a perfect hole. I then mark the case with a bright paint marker in a conspicuous way so that it doesn't get loaded.