Checking case capacity, Grains of Water

[vaguru]

I learned to check case capacity in grains of water by seating a bullet to desired COL in a sized unprimed case. Weigh the bullet case combo, then using a hypodermic needle inject water to be flush with the flash hole. Surface tension will hold the water in place. Now weigh the combo with water. Subtracting the empty weigh from the filled weigh gives the capacity in grains of water.

To check other brands of cases, do the same thing and then you have a valid comparison. I learned this from the old Powley Computer 30+ years ago.

I don't see anyone using this method currently and was wondering why?

I have used the Powley Computer to calculate loads that weren't currently available, and years later when published load data arrived, I was right on. Unfortunately, The Powley Computer only serves for IMR powders of the time, and current powders make it obsolete for that purpose.

However, the method of calculating case capacity of water, to a seated bullet, remains valid. I believe this is the only method that gives true case capacity of water, with ones selected bullet of choice.

Having this information, one can compute the different case capacities, and adjust load data for cases not specified in published data.

[Klem]

Vag,

With respect, that method is flawed for at least the reason that bullets have different shaped bases. Different shaped bullets and different seating depths displace different amounts of water.

If using programs like Quickload their water case capacity assumes no bullet and full to the SAAMI case mouth (upright). The difference between a concave or convex [water] meniscus is too minor to be concerned about. I seal with a used primer in a sized case to do this, and take a group average of cases.

[vaguru]

Klem,

When using this method one uses the SAME bullet and SAME COL. Then the comparison is just the difference in the weight of water in the cases.

[vaguru]

To end my curiosity, I just went through this process with Hornady and PPU current cases. Both cases were full length sized, trimmed to 1.510" and deburred inside and out.

Then I seated a Hornady 120 gr ELD Match bullet to a COL of 2.200", the length for my new barrel. This was now weighed on an Ohaus 10-10 scale.

Empty weight with bullet seated 236.5 gr. Now case was filled with H2O using a hypodermic needle to fill the flash hole but not the primer pocket. Tapping to make sure all air bubbles are removed.

Filled weight with H2O 264.6 gr. Subtracting the 2 weights I ended up with 28.1 gr H2O.

Now using a current PPU case with small primer.

Empty weight with the SAME bullet (pulled from the Hornady case with an inertia puller) 241.7 gr. Filled with H2O using the same procedure as above.

Filled weight with H2O 269.7 gr. Again, subtracting the weights I ended up with 28.0 gr H2O.

The PPU case has 99.644% capacity of the Hornady case. Thus, with any loads currently developed for the Hornady case, a charge of 99.644% of that amount will give very similar results when fired. For all practical purposes, the same powder charge can be used in either case, unless of course you are loading max charges, then I would still work up.

I was surprised that the capacity was this close between these two cases as the PPU case was heavier by 5.2 gr empty. Apparently the extra weight is on the outside of the case, and not deleting internal capacity.

My curiosity satisfied, think I'll go load some ammo.

[Klem]

Vag,

Quickload calls what you are calculating 'Usable Case Capacity'. It calculates this based on either the default 'Max Case Capacity' (35.99gns), or what you have input from your own testing of upright empty cases.