What internal ballistics determine velocity?

[Scott Bear]

There is a Berger document about COAL and it states that if you seat a bullet long you can fit more powder in the case and get a higher velocity for the same pressure.

Is this statement accurate? I thought pressure was the determining factor for velocity? How would you get a higher velocity if the pressure were the same? Are there other forces at work (total amount of energy or power) that determine velocity?

 

[adamg]

It's mainly the area under the pressure-time (or pressure-bullet position) graph that dictates velocity.

By seating the bullet longer, you get more case capacity and can then add a bit more powder for equal peak pressure as the shorter scenario. But because there's more powder with the same peak pressure, the graph will mostly be higher pressure throughout, resulting in more velocity.

Gun designer and manufacturer Mark Serbu in USA did a video that covers this general discussion, though not your exact question about seating depth. But it shows many examples of this graph I am referring to and how it yields velocity.

 

[Ganderite]

Area under the curve. More powdrr means more energy, But this will increase peak pressure unless you do something else. If bullet is 50 thou off rifling, moving it forward to 20 thou migh reduce the peak. But if you get too close to the rifling, peak pressure will rise again.

If you hook an oscilloscope to your transducer, you can see the pressure for each shot, like this:

 

[bearhunter]

This got me thinking, big mistake, back to some articles in Handoader magazines.

HL, 1986, sept/oct, Issue 123, Page 23 there is an article by Dave Scovill that goes into a bit of depth on interior ballistics and he includes a few formulae

HL, 1994, may/june, Issue 169, Page 22 there is an even more indepth article on Internal Ballistics, with pressure graphs and explanations, written by Andrew Lambley.

They're both a bit dry reading, for those not inclined to do the math but the graphs and explanations are quite informative.

If you have access to these articles and are interested in Internal Ballisitics, I believe they're online somewhere and many have the Handloader/Rifle issues on CDs or have downloaded their program.

 

[bearhunter]

Area under the curve. More powdrr means more energy, But this will increase peak pressure unless you do something else. If bullet is 50 thou off rifling, moving it forward to 20 thou migh reduce the peak. But if you get too close to the rifling, peak pressure will rise again.

If you hook an oscilloscope to your transducer, you can see the pressure for each shot, like this:

This is an old trick used by many bench rest shooters, using small case capacity rounds.

They don't have as much neck tension on these loaded rounds and because powder capacity is limited, the bullets are seated in a manner that their ogives will not only be jammed into the lead but it's preferrable if this actuall pushes the bullet back into the neck a few thou.

There has been some pretty heated controversy over this practice by some, but when I was still in the game, it was a pretty common procedure.

 

[Scott Bear]

Interesting stuff. I'll have to do more reading.

So the area under a pressure-time curve is the pressure impulse? Like a blast wave?

I always thought of it like this: the area under the curve of an energy-time graph was power, power caused pressure and pressure caused the bullet to move. That's probably an over simplified and ignorant description.

But still, how could you keep the pressure the same and increase speed? Increase the impulse? Wouldn't this take more than just increasing the case volume?

 

[adamg]

But still, how could you keep the pressure the same and increase speed? Increase the impulse? Wouldn't this take more than just increasing the case volume?

When people talk about a pressure number, it's a single number, specifically the peak pressure number. This peak is what is doing the damage - to actions, to brass, to primers, to bolt lugs.

So the way to increase speed without increasing peak pressure, it to keep the peak pressure the same but have more pressure for more time while the bullet is in the barrel. In other words, more area under the curve, but the peak is the same.

 

[Chilly807]

Think of it this way. Referring to Ganderite's graph, the maximum height or level of the pressure peak should stay the same, but is extended to the right over a longer period. Same max pressure, but extended over a longer period of time, equals more velocity.
This is assuming you've got powder with the right burn rate. If it's too fast, increasing the amount in the case will cause higher pressure but not much more speed. In other words, a sharp spike, then little sustained push.

 

[Juster]

I always thought of it like this: the area under the curve of an energy-time graph was power, power caused pressure and pressure caused the bullet to move.

Power is energy/time slope, my understanding is that the area under the curve of an energy-time graph is energy times seconds, or work. I could be wrong.
Edit: wrong, energy is work, so it energy times time, which isn't really anything.
Edit: Just realised that it doesn't really matter, excuse me, carry on.

 

[Andy]

You're all skirting around the answer.

I could make it sound more scientific, but it's simply:

more powder = more energy = higher MV