Some folks operate on intuition or instinct. Not always bad, but sometimes it leads one astray. As long as there is enough pressure in the bore the bullet is accelerating. By "enough" I mean an adequate pressure exerting a force on the bullet base that's greater than the opposing force from bullet inertia and friction in the bore.
Basic physics; if there's a net positive force on the bullet it will accelerate. Force = mass * acceleration.
Quickload gives insight to the amount of powder burned in a bore. It can be surprising. Many rifle loads in typical barrel lengths burn 100% of the powder charge, yet the bullet has a net positive force on its base as it exits the muzzle. So despite burning all of its powder, the bullet velocity increases with barrel length increase. Slow powders in short barrels don't burn all their charge.
A small charge of fast powder in a very long barrel >may< result in slower velocities with exceedingly long barrels. I surely don't know this, but there are accounts from years past of somebody taking a long 22 LR barrel and cutting it bit by bit. The results indicated anything longer than 16" or so yielded slower velocities.
This is my 20 m$ based on a lifetime of university studies and an engineering career. Feel free to argue with me. When I'm wrong, I want to learn from my errors.
Not wrong, not completely right. And then there are low velocity loads.
Using a 30” barrel and a 3000 fps muzzle velocity for example. Simple math says the bullet is accelerating at 100 fps each inch the length of the barrel. We know that’s not true. It’s at acclerated 2500 fps at 15” then rule of thumb accelerates 30 fps per inch for next 15”. Also not exactly true, but makes the explanation much easier.
There is a point for every cartridge like the example of 22LR. It’s just a different length barrel. When the powder burns out, gasses are still expanding, increasing velocity. The problem is the point where that acceleration is minimal. When you need 10” of barrel to gain 100 FPS, it’s not worth it. This is what I meant in my earlier post “acceleration stops” it is so little it’s meaningless. The bullet hasn’t stopped, only stops accelerating. Almost every cartridge out there has a point where people know, more barrel isn’t the answer.
Super sonic rounds plot on a graf as a steep rise to peak pressure to an ever decreasing upward slope angle. No on shoots a barrel long enough to see the fall off and eventual stop. I’m not aware of anyone plotting squib loads.
Subsonic is a different issue. What you see is the same load in an 8” barrel is 950 fps, 1150 in a 16” and 1000 fps in a 24”. Some of this can be attributed to different barrels, but when it happens enough, you have to acknowledge the pattern.
The hard variables to calculate are bearing surface of the bullet, true diameters of the lands/grooves and condition of the bore. Never mind the changing of the burn rate of the powder in a less than optimal density. Subs are no more than a controlled squib and why it’s really suggested that you work loads down in velocity, not up. Way too many bullets lodged in barrels by too many people over the years to not take the possibility seriously.
Bottom line, there really will be no hard and fast answer to the original question. Change the powder or bullet weight and the answer will change.
Subs require a little bit of a thinking adjustment. Or engineers need to step outside the classroom once in a while. You’ll get new problems to solve.
). This is the reason most mouse fart loads are pistol powder. They ignite quickly and all at once no matter what you do with it.
