I'm not looking for reduced loads. I want to see if a little faster burning powder would increase the velocity lost from a radically shorter barrel...
This
No it will not
Not always true. Depends on how much barrel you are lopping off.
Are you sure about that? That would imply that in some cases a longer barrel may actually be slowing a bullet down. Doesn't sound right.
Rocky provided a good explanation of some of the physics, maybe that's because he's a rocket scientist, but let me expand a bit.
Changing the burn rate changes the shape of the pressure curve. If we load two different powders to the same maximum pressure, both curves will have the same maximum amplitude, but changing the burn rate change the shape of the curve. These curves are not nice sin curves, but something like this, with the red line representing pressure and the blue line representing velocity:
[img]
https://www.shootersforum.com/threads/pressure-curves.70991/#lg=thread-70991&slide=0[/img]
Looking at the above graph, think of the area under the red line as the amount of work performed against the bullet over a given length of barrel. As we increase the powder burn rate, the area under the curve shifts to the left. In other words, more of the work that adds velocity to the bullet is performed in the front part of the barrel. With a slower burning powder, the height of the pressure curve doesn't tapper off as quickly, and longer barrels are able to take advantage of this relative pressure differential for a longer time than shorter barrels.
Additionally, certain cartridge geometries benefit more from a longer barrel/slower powder dynamic. In general, the more over bore a cartridge, the more additional you will notice the differences in the impact of barrel length in both velocity increase due to added barrel length and changes in optimum powder choice relative to barrel length, but again, these are generalities.
With all that said, we must recognize the practical truth behind the claim that barrel length makes no difference in relative velocity potential of different powders. In the context of hunting length barrels, say 20 to 26" inches in length, this is mostly true. At the theoretical level differences do exist, but for the average reloaded, these will be masked by margin of error inherent in the small sample sizes he chronographs.
