From: bartbob@aol.com (Bartbob)
Newsgroups: rec.guns
Subject: Re: brass tales
Date: 29 May 1997 20:07:25 -0400

If one uses a pressure measuring system to compare identical loads in two
different cases where one is 10% heavier than the other, they'll find
higher pressure in the heavier case.  But that's grade-school physics;
burn the same amount of fuel in a combustion chamber with less volume than
another and the pressure is higher.

The appearance of these two fired cases may look identical, especially if
the highest pressure is at or just below maximum for the round.  And if
the weight difference is less than 10%, peak pressure differences will be
less and not as easy to detect.

I've used .308 Win. cases weighing 147, 157, 167, 177 and 187 grains.
With the same component suite, neck tension, bullet seating depth and
barrel tested at the same sitting, muzzle velocity increased with case
weight about 25 fps for each 10 grains of case weight.  If the pressure
wasn't higher with the heavier cases, their muzzle velocities would be the
same as the lighter cases.

BB

From: Norman Johnson <njohnson@nosc.mil>
Newsgroups: rec.guns
Subject: Real Difference in Brass?
Date: 27 Dec 1997 10:07:54 -0500

Someone wrote:
#>> # Will their ballistics and performance really be that much different?

Military brass is usually a little heavier,perhaps by about 5-10 percent.
This is another gun writer old wives tale - that because the military brass
is heavier and therefore has a lesser internal volume so must be loaded with
less powder.  It has been years since I have done it but my calculations
showed extremely small differences in volume with the largest weight
variations that I could find.  A ten percent weight difference does not
translate to 10 percent volume change.

Andrew L. Brant" <abrant@BRL.MIL> wrote:

#Doing some quick estimates I come up with a 6% change in case
#weight is needed to change the volume by 1%, assuming the outside
#of the cartridges are the same.

To the doubting Thomases, take an old case and file off enough brass to
weigh 15 grains which is about the maximum variation that I have found for
.30-06 cases, less for the smaller ones.  That should be pretty convincing.

Other factors can effect pressures enough that brass weight variations are
down in the noise.  For instance, according to my calculations, setting the
bullet .1" deeper in the .308 case reduces available case volume by about
4%.  Many reloaders experiment with varying bullet seating depths much
greater that that without giving it a second thought and certainly
experience no pressure excursions.

God Bless!

Norm