How to determine, internal case volume?

I found something interesting with a subsonic load. I am thinking I might have a significant difference in case volume for some of the brass.

Or I have something else going on.

An easy way is water weight filled to overflow, deduct the weight of the empty case from the filled weight, or zero a digital scale with the empty case on the scale, then fill the case with water and weigh it again.

quote:

Originally posted by ddunn:
How to determine, internal case volume?

I found something interesting with a subsonic load. I am thinking I might have a significant difference in case volume for some of the brass.

Or I have something else going on.

The water method is the best, because case volumes are generally given in grains of water. But for a dry method, I use H335. I fill the case in question to the mouth, then compare that with a standard case, such as the .30/'06 to see if the case being measured holds more or less powder. For example, the .30/40 Krag cases I am currently using hold 87% of the fully charged .30/'06 case......

I've used a hypodermic needle when using he water method. I seat the bullet to correct ORL and then
use the hypodermic needle to fill through the primer hole. Seems to work pretty well. I've had to do some explaining to the pharmacy as to why I need the needle but once explained, I've never been turned down.

In my confused mind, I think using water to determine case volume is not the way to go. Consider: Water has surface tension. How certain are you that you have gotten all the water out if you fill, then measure? I motion that you try salt. It will completely empty out of the case if you insert the primer upside down, and you will not have the possibility for damage to your electronic powder scale if you were to spill the sample on your bench. You could even use the same sample of salt over and over to determine the capacity of subsequent cases relative to Case Number One without having to handle sloppy water.

HB
Weight a empty case/primer. Now either tare out your scale, or write down the weight. Now fill that case with water, and weigh it again and do the math.
There are many things you can add to water to reduce what your worried about, the stockcar guys like "water wetter", photo developers have a "rinse aid" they add to the rinse water (couple drops/quart), or you could look around the house for dishwasher rinse agent or even plane old fashioned dish soap. All of the above reduce the surface tension of the water.

Besides, with a water weight measurment, anybody can check/verify/compare your results to their results, with out worrying about the different materials (IOW not everyone has the same powder you do, has the same size salt crystles you do, etc, but fresh water is the same around the world).

Hey ddunn, Though you asked specifically about Internal Volume, you may be able to achieve what you want without the water at all.

If your Cases are all from the same manufacturer and from the same Lot, you can do the following with the Cases you found something "interesting" with:
1. Just weigh them and use other Cases from the Lot which weight within 0.5% of the overall Case Weight.

You do need to perform a complete Case Prep on them so they are as Physically alike as possible. But that should have been done on the complete Lot before you started reloading any of them. If you did not do it, you can still do it and the only minor variation will be the Powder Residue remaining inside the "Fired" cases.

The "water" will absolutely give you Internal Volume in relation to other cases filled with water. But, you need the Cases to all be once-fired with a Spent Primer in them to retain the water. I suppose you could put Spent Primers into "New" cases and measure the water. Once you do a couple, the mess and hassle factors will be enough to see why I recommend just weighing the fully prepped cases.

Best of luck to you.