What is meant by "useful case capacity"?

[Lee Roder]

Interested in load density and I thought I knew but referring to Lee's Modern Reloading (2nd ed) pages 561, listing the 38 Special's useful case capacity as 1.30 cc, and 569, listing teh 38 Special +P's useful case capacity as 1.18 cc, apparently I do not.

I thought the cases were identical except for perhaps optional headstamp designation. I've been loading them interchagneably.

 

[Steve C]

In this case one of the measurements is simply wrong. Books often have entries that are wrong due to a misprint or improper entry that's not caught by proofreaders.

The +P and standard velocity cases are identical as you stated. The +P being for load discrimination only. Unless the book is referring to the capacity with different weight bullets its just wrong.

 

[Kernel]

I'd assume what Lee means by "useful case capacity" is the volumetric capacity of an empty case, measured to the rim, minus the volume taken up by a seated bullet.

In literature this is sometimes referred to as Effective Case Capacity, or Loaded Case Capacity. All else being held constant, it's a function of seating depth.

Assuming it's not a mistake, the only logical reason there'd be a difference between the two (.38 Spl & .38 Spl +P) is if one used a heavier (and therefore longer) bullet, given they both had the same COAL and empty case capacity.

Load Density, usually expressed as a percentage, is a function of the amount of empty space between the bottom of the seated bullet and the top of the powder column.

 

[USSR]

..."useful case capacity" is the volumetric capacity of an empty case, measured to the rim, minus the volume taken up by a seated bullet.

+1.

Don

 

[helg]

the only logical reason there'd be a difference between the two (.38 Spl & .38 Spl +P) is if one used a heavier (and therefore longer) bullet, given they both had the same COAL and empty case capacity.

+P case is designed to operate under higher pressure. Its brass has to be thicker, especially on a primer side. Thicker brass for the same external dimensions leaves less internal space under a bullet, which is the "useful capacity".

 

[Lee Roder]

Thanks, some unstated bullet volume does seem a plausible explanation.

I got curious as to what value I SHOULD be using, so I decided to determine "useful case capacity" myself for 2 pieces of sized brass (in case they differed):

IK 03 (1.151" 65.08gr) - 1X fired
Winchester (1.151" 64.66gr) - god only knows how many times this one's been fired

My dremel cut a slight groove down the side of each of 2 bullets (Dardas LSWC nominally 158gr) and dead primers were seated into each case. Each case was then filled with water and these bullets seated to various depths.

Room temperature here is 80F this norning so the density of my water 15.38 grains/cc

IK03+Dardas(158gr) 224.28gr (empty)

OAL grains H2O CC
1.645 243.14 18.86 1.23
1.622 242.48 18.20 1.18
1.591 241.90 17.62 1.15
1.564 241.22 16.94 1.10
1.537 240.54 16.26 1.06
1.512 239.94 15.66 1.02
1.485 239.26 14.98 0.97
1.461 238.70 14.42 0.94 <<<
1.434 238.02 13.74 0.89 <<<
1.410 237.34 13.06 0.85
1.385 236.82 12.54 0.82
UsefulCaseCapacity(cc) = 1.58 - 1.38 * OAL(in) (rho > 0.9997)

WINCHESTER+Dardas(158gr) 224.18gr (empty)

OAL grains H2O CC
1.644 243.18 19.00 1.24
1.621 242.62 18.44 1.20
1.590 241.92 17.74 1.15
1.563 241.18 17.00 1.11
1.535 240.46 16.28 1.06
1.509 239.80 15.62 1.02
1.481 239.10 14.92 0.97
1.455 238.52 14.34 0.93 <<<
1.427 237.86 13.68 0.89
1.403 237.22 13.04 0.85
1.378 236.72 12.54 0.82
UsefulCaseCapacity(cc) = 1.60 - 1.39 * OAL(in) (rho > 0.9996)

Now I've been seating my range ammo to an OAL of 1.455", providing a "useful case capacity" significantly less than the 2 figures given in Lee for unknown bullets, IOW a load density significantly above published.



How much variation in load density for a given powder charge is "OK"?

Is say a 20% difference in load density which arises from 20% more powder ballistically equivalent to the same change in load density arising from deeper bullet seating?