One would not expect the payload velocity vs time plot to look like the pressure vs time plot. The pressure against the base of the wad is converted to force (pressure times area of the wad base; force pounds per square inch times square inches equals force pounds). Force equals mass time acceleration. The mass is fixed, so the acceleration will vary proportionally with the force. So, the acceleration curve will look like the pressure curve....
I am not sure this is true. It has long bothered me that there are no internal ballistic programs for shotgun with the ability to predict internal pressures. They certainly exist for rifle and handgun loads. One of the best in that arena is QuickLoad. (Although it does seem to have it's weaknesses.)
I reread Ackley, Powley and Hatcher, whose pioneering work lead to the formula's that are used by today's software.
There is a major impact upon internal pressure when the bullet encounters the rifling. The shape and composition of the bullet directly impacts pressure. To more limited extent seating depth and type of seating of the bullet play a role as well.
QuickLoad maintains a database of powders and bullets. For bullets not in the database you must make detailed measurements and enter them into the software.
The culprit here is friction. As the projectile requires more energy to be driven down the barrel, internal pressures will build.
A shotgun shell is even more complex. That is why most reloaders follow a recipe so "religiously". If you change the wad, you change the friction, which impacts the pressure.
Also, this group is probably more aware of internal pressure issues than most around, because of the age of the guns we are usually talking about. My understanding is that internal pressures in a shotgun peak about 16" from the chamber. So has the maximum velocity of the projectile been reached within the barrel?
Pete
