For some cartridges, better headspace consistency, sure. In most cases all the gains are from raising
pressures, not from "more efficient shape" which I think is about 98-99% myth, over the
range of normal modern bottleneck cartridge geometries.
Could you invent a 4" long .22 straight case versus a 2" long bottleneck shape where the combustion
chamber shape would make a difference? Sure, maybe. But a tiny bit of shoulder shape diff
that many have make some significant diff in burning efficiency? I am very skeptical.
Barsness has tested some short fat modern carts against long skinny old carts......nothing to be found
just in the shape, it seems. Why do you think pretty much all of the hot new SSMag and such are all
just gone, most (all?) no longer cataloged? Because they don't actually do anything better than the
cartridges that we already had.
But better, more consistent headspace because of a better shoulder shape is a real issue for some cartridges,
and some of the AIs have very significant case volume increases, which is not bad. But the ones with a ~5%
volume increase where the parent cartridge has a nice shoulder size and angle......probably little or nothing there.
Look at the powder charge that a 7mm Mag needs to push the same bullet as a 7mm-08 just about 10-15% faster.
140 gr 7-08 uses 46 gr of W760 to make 2800 fps, the 7mm Mag uses 60 grains of the same powder to make
just under 3000 fps. 30% more powder required to make 7% more velocity. Now with slower powders, and
much more of them, you can go faster, a compressed charge of IMR8133 78 gr will make 3200 fps. So now
we have 1.70 times as much powder to get a velocity gain of 14% in velocity.
So, if your AI will let you burn, say 10% more powder (and I think most AIs do less, but haven't looked in
great detail) if 30% makes 7% more velocity, I'll bet 10% more will make about 2.33 % more velocity (at the same
pressures).
The 70% increase of a slower powder gets 14% more velocity, so using that ratio, 10% more
powder space might get, 1/7th of 14% ----2% more velocity at the same pressures. I imagine that AI folks,
KNOWING that they have to have higher velocity, just pump up the pressures and get that
expected gain mostly from pressure increase.
Hate to not believe in the AI Easter Bunny, but the laws of physics and all.....
Bill