Amount of Powder and Burnout Point Question

[Mr_Flintstone]

I was doing some simulations today for a couple of .357 Magnum rifle loads, and I noticed something I had never seen before. It seems that when I decreased the amount of powder for the load, it moved the peak pressure point and powder burn out point closer to the muzzle (or after the bullet left the barrel), and when I increased the load, it moved the peak pressure point and powder burn out point closer to the breech. I tried it with a few different powders to see what the effect was, and in each case, it was the same.

Is this really the way it works, and if so, why?

[Goofy]

Smokeless powder burns more quickly as pressure increases. Bit of a vicious circle one might say. The inverse is true as well. Less pressure, slower burn rate, flatter pressure curve.

[mehavey]

I was doing some simulations today.......with what model?

[McFred]

Is this really the way it works, and if so, why?

Yes.

Chemistry/physics:

pV=nRT

e.g Ideal Gas Law (https://en.wikipedia.org/wiki/Ideal_gas_law)

For a given load, you can offset/compensate the peak pressure by pushing the bullet back into the case for a shorter overall length (changing the 'V' in the formula above). This is why people might be concerned over bullet "setback" regarding high pressures and guns not built to safely withstand pressures well in excess of the cartridge design.

[Mr_Flintstone]

...with what model?

I was using Gordons Reloading Tool to check 125 grain jacketed bullets with Bullseye,Unique, Titegroup, and 231 in a 20” barrel.

[BigAlofPa.]

Something i learned from Donny summers that helped me with powders burning better. And leading. I asked if he makes 12 BHN boolets for in my sw 25. It was a simple fix. He said don't be afraid to give a heavy crimp. And it worked i didn't get any leading.

[mehavey]

As mentioned above, most modern powders are progressive burners.
"Burn Rate" is only starting burn rate.

They burn/consume themselves faster as pressures rises.
https://thefiringline.com/forums/showpost.php?p=6624170&postcount=15

While the powders may burn out sooner at higher load pressures, they
still increase the area under the pressure curve associated w/ bullet
travel in the barrel. It's then the total AREA of the curve that does "work"
to acceleratenthe bullet -- hence increase velocity by the end of travel/muzzle.

[oley55]

I was doing some simulations today for a couple of .357 Magnum rifle loads, and I noticed something I had never seen before. It seems that when I decreased the amount of powder for the load, it moved the peak pressure point and powder burn out point closer to the muzzle (or after the bullet left the barrel), and when I increased the load, it moved the peak pressure point and powder burn out point closer to the breech. I tried it with a few different powders to see what the effect was, and in each case, it was the same.

Is this really the way it works, and if so, why?

Curious if your simulations involved IMR4227. I had some serious keyholing at 100 yards while trying to work up loads for 185gr, BTB FNGC bullets in a 24" Rossi using IMR4227, (13.0-14.0gr) I didn't stay with it to see if max or max+ loads would work because H110 was working with all loads. I was surprised 4227 didn't do well, as it was/is my impression IMR4227 should have excelled out of that long barrel.