Internal Ballistics

[918v]

How much pressure does it take to maintain the velocity of a bullet in a rifled barrel? How would you calculate that?

The reason I ask is I'm working up a load for a 9mm carbine, going up in .2gr increments, and there is a 100 FPS jump in velocity between 3.8 and 4.0 grs of powder but only a 40 FPS jump between 3.6 and 3.8grs. Now, you're thinking 3.8 is max as shown by the jump in velocity. But no. Max is 5grs.

I'm thinking the 3.6/3.8gr loads run out of steam in a 16" barrel and are decelerating out the muzzle while the 4gr load is still accelerating out the muzzle.

A friend of mine told me no, the bullets are not decelerating out the muzzle but rather the efficiency of acceleration has been compromised. But if that were the case then the rate of acceleration would not have dropped off so abruptly.

I have QuickLoad but it's too stupid to figure this out.

Please help!

 

[mikecr]

I don't think a bullet would decelerate in a barrel.
There is still plenty of pressure behind it, even if the barrel was 160" long.
QL shows this.

 

[918v]

QL also shows the bullet accelerating out of a 400" barrel with 15 PSI of muzzle pressure.

But at what point does barrel friction overcome pressure?

 

[Lapua40X]

QL also shows the bullet accelerating out of a 400" barrel with 15 PSI of muzzle pressure.

But at what point does barrel friction overcome pressure?

Finding the amount of friction needed to overcome the pressure is not a linear calculation. I suspect you're going to need to plug static friction coefficient into your calculations to come up with the answer to that one. In your experience, you seem to be expecting the increase in MV to be linear - it isn't; and even though you increase the load (and thereby the pressure) there is a point where the bullet has to leave the muzzle before all of the pressure available in the charge is fully developed.
It might be enough to know that as the bullet moves toward the muzzle the volume of space within the enclosed bore between the bullet and the chamber increases. As volume increases, pressure decreases and eventually there isn't enough pressure to overcome friction so the bullet stops its forward motion. Of course, it would have entered a state of deceleration long before that point. But don't worry, your bullet isn't going to slow down while it's moving down the barrel and it's velocity will actually increase slightly after it leaves the muzzle.
The powder charge in your cartridge doesn't explode, it burns at a rapid rate. The initial point of ignition creates enough gas to move the bullet out of the case and into the bore. But the value of the energy doesn't reach its peak instantaneously, it continues to develop as the powder continues to burn and the bullet moves down the bore. The initial ignition provides the energy necessary for the bullet to overcome the friction that held it in the case neck and the continuous burning of the powder provides an increasing amount of energy for the bullet to overcome the friction of the rifling and accelerate toward the muzzle where, as it exits, the unused gases are exhausted around and behind the bullet.