Internal case capacity effect?

[Kiwi cop]

I recently loaded 500 rounds of 45ACP using new Starline cases and a max load of 4.8 gn Tightgroup under a 230 gn JRN projectile. I have yet to fire any of these loads as they were loaded for use at our IPSC Nationals next week, but I have fired and sighted in identical loads in some mixed brass I have been using for a couple of years, mainly Winchester cases.

The first 300 cases were nickel, and I visually checked each powder drop before placing the projectile into the case neck.

The last 200 cases were ordinary brass as I could not get any more nickle cases , and when visually checking I saw the powder level was slightly lower than in the nickel cases. I put this down to thicker case walls and a lower case capacity due to the nickel plating.

A few days ago I read some specs on Starline 45ACP +P cases, noting that they had an internal capacity about 2 gns less than a standard case due to thicker case walls and base.

This has me thinking of what is the effect on case pressure due the difference in case capacity between brass, nickle and +P?

Any ideas?

 

[s&wchad]

Decreasing the case capacity increases pressure. Hodgdon lists 4.8 gr's of Titegroup as the maximum load with a 230 gr Hornady FMJ FP bullet, using Winchester cases. Changing any of the components can impact pressure.

There's a reason it's SOP to work your way up to maximum loads. Having said that, Starline is excellent brass and I doubt your load will harm a modern firearm in good condition. If I was loading mixed brass, I'd probably back off to 4.6 gr's.

 

[BLUEDOT37]

A few days ago I read some specs on Starline 45ACP +P cases, noting that they had an internal capacity about 2 gns less than a standard case due to thicker case walls and base.

Over the years I've measured new case capacities of many different cases & my notes show regular S-L 45ACP brass hold 27.3gr H²O Wt. while their 45ACP (+P) cases hold 26.4gr H²O Wt.

Any decrease in capacity will affect pressure but this small of an amount & at 45ACP pressures shouldn't be material.

I've also noticed routinely that nickeled cases differ from their brass siblings in weight & capacity.

.

 

[stansdds]

Reduced internal case capacity will result in higher operating pressure and velocity, guaranteed. With plinking loads that are well below maximum safe pressure, it's usually not a problem, although point of impact and accuracy may be altered. When loading to maximum safe levels or loading to a specific velocity standard, varying case capacity becomes an issue.

 

[WR Moore]

AMEN to the post above. I recently got a lesson in applied physics over capacities at least as judged by case weight while at the upper end of load data. I found that Federal Gold Medal Match .308 brass is similar to LC Match brass (made when Federal ran the LC plant), but item to item variation is much greater and seems to need 1 gr extra of powder for the same result. Oddly, LC ball brass is similar to LC Match, but not identical and the weight variation is also greater. IMI "commercial" brass is pretty close to LC Match. Case capacity may also vary by production run and the materials used.

Actual commercial brass has wide swings between brands despite the fact that they're supposedly working off the same drawing. FWIW, the one box of Black Hills ammo I bought has case weight variation that would delight benchrest shooters. As does the Lapua Palma brass I recently bought (can't find LR primers, buy brass that uses the primers available). Gorgeous brass, needs +1.5 gr of same powder to run same as LC Match.

At the pressure levels of the .45 ACP I wouldn't worry about pressures, but you might want to run a few rounds over a chronograph to make sure you're making velocity with some room to spare. Like Kiwi, I've used a bewildering assortment of brass over the the decades without issue, but I wasn't trying to just barely make the power factor.