Home Forums Reloading Big Game Rifles Best way to determine case capacity???

I have begun working with a 7mm Weatherby in a Mark V action and have come up with an accurate load off the bench, but had an experience today that has me a little concerned.

I took said load out to chronograph it so I could get my CDS cap from Leupold ordered and I ran three over the screens with my chosen load with no problems other than a little more deviation than I would like to see.

Next I decided to run a load .5 grains lighter through the screens. It booked 3,127 when the average of the other three was 3,020 and it was as close to a case head separation as you can get without actually separating.

I got to thinking about cause and effect and here is what I know. All of my brass is Weatherby brand and properly stamped to the caliber. What I don't know is how many lots of brass might be mixed in together, and how old some of it might be. I got some of my brass from a brother-in-law that might be pretty old (and I think I can now safely assume reloaded a few times).

I know how to check for an imminent case head separation with a paper clip straightened out, so I can filter through those for scrap, but am concerned about the good brass being different capacities after what happened today.

Best way to sort...weight, capacity, other?????

Anything else I should be looking at???

Thanks for you help in advance.

I start over with new brass so I knew what I had....and just load up the old stuff with zombie loads.

"The Best", most accurate sort for absolute uniformity would be by case capacity. It is a slow way to sort brass.
If I decide I need to sort, I sort by weight of fully prepped brass, which is close enough for me.
Sometimes I will take ten cases of the same weight and check water capacity; I've never found enough variance to make me switch from weighing to checking capacity.

You don't have to use water to check capacity. You can use fine sand or a fine ball powder. I use A2230 powder. No wet cases to dry after checking.

An old timer told me this...center fire rifle

To determine 100% capacity with the bullet and powder combo you intend to use

Take a fired piece of brass...do not size...the bullet needs to fit loose with no tension

Fill the case with powder to just shy of the bottom of the neck then drop in the bullet.

Measure your over all length with the bullet sitting on the powder.

Add or remove powder until you reach the target OAL

This is your 100% case capacity with no compression with that powder and bullet

Do this for several cases at random throughout the batch to find an average

Given that, you're wasting your time and components.

Get a new batch of brass and keep it segregated into sub-batches each with its own, known work history.

If your cases are all the same length, and otherwise the same external dimensions, weigh them dry. That will be real close for internal volume also. Sort to within plus or minus 5-10g. You won't have more than 2 or 3 grains of water weight difference which is pretty close.

If you anneal them all, they should all be pretty consistent and usable until you get new cases, which is your best option.

In my experience, cases from the same lot will be within 1-2 grains of each other. Anything more than 5gr out should be culled. You can also verify capacity using fine ball powder as described. Use fired cases and leave the spent primer in or reverse seat a spent primer to take the primer volume out of the equation.

The best is water with all cartridges in the same state (best is fired, but not resized so all match ones chamber dimensions). Weigh dry and then filled with soapy water. Soap or detergent will help eliminate a potential meniscus issue (i.e. water clinging either above or below the top of the cartridge neck). Degree of fill must be consistent. Determine water weight by difference and convert to volume.

I've personally gone to simply weighing once fired and then anal prepped brass.

Like said above, if your cartridges are pooled from mixed sources, I'd be inclined to dump and start from scratch.

another possibility for the extra pressure and separation of that case is foreign matter in your case. i have had cleaning media clump for some reason inside a case, effectively reducing the capacity. had a 284 winchester failure that i surmised was from that after pulling apart others of that batch. lesson learned, now i check each cartridge inside. made easier now by using a steel pin tumbler.

What else you don't know is how the mixed brass has been sized.

If it's been fired a few times & sized incorrectly with the shoulder pushed back a few time & fired, the case has very likely stretched & weakened ahead of the belt.

The advice to get new brass & start over is the only solution & then when re-sizing, just size down enough to allow the case to chamber & do not push the shoulder back even though it's a belted case.

After multiple firings, you may also want/need to eventually buy a collet sizing die to further size the case body just ahead of the belt more than is possible with a standard sizing die.

Collet Sizing Die for Belted Brass

MM

Temperature affects pressure, and with temp sensitive powders, that can easily cause pressure spikes, but case separation is a different issue and is mostly not related to pressure, it's related to headspace.

The first step with belted brass is to carefully check each case internally for the stretch ring right above the belt with a bent paper clip. Sometimes you can use a small flashlight to peer into the case and the stretch ring can be seen. Recycle any that are stretched.

Next you can weigh each case and see if there are any dramatic differences in weight that would account for pressure variations.

With close inspection, you may be able to sort the brass further by looking at minor changes to the headstamps, but once you make the cases as uniform as possible and anneal the necks, you should have a usable batch of brass that's good to go.

Temp-insensitive powders help to avoid pressure spikes, and keep in mind that the Wby has enough horsepower on tap that it doesn't need to be hot-rodded to perform well. If brass life is a concern, hot loads can kill a case quickly.

Weatherby brass is readily available. I know some guys who only use new or their own once fired brass for their more serious endeavours.