pressure vs velocity

[jim8115]

Supposing you have a load with published, safe data. In this example a 180 gr cast for a .40 S&W. If you need to substitute a different profile ( but same weight ) bullet that may be longer, or need to seat deep to accomodate a certain gun , is it safe to assume that as long as the velocity is within the published range that the pressure is also? Assuming same powder , of course.

In other words, if I have a load that is 900 FPS according to the book, and verifies with a chrono, and I change bullets / seat deep, drop the powder charge to allow for it, and it is still 900 FPS, should the pressure be the same?

JIM

 

[tightgroup tiger]

I would certainly think so, I use that same philosophy with my loads.

 

[Zak Smith]

No, that rule is not hard and fast.

Here are two scenarios I ran in QuickLoad:

Case 1. .40SW, 180gr XTP (bullet length 0.661"), COAL 1.135", 4.6gr N320, 989 fps @ 32482 psi max (4.5" bbl). Usable case volume 0.591 cm^3

Case 2. Same everything but set bullet length to 0.681, this reduces usable case volume to 0.549 cm^3. I reduced the charge to 4.49 gr to bring the velocity back to 988 fps, but this put the peak pressure at 34155 psi.

 

[Walkalong]

As posted, the bigger factor is case capacity for the initial burn. I log all bullet lengths as well as cartridge OALs so I can figure how much deeper in the case one bullet is over another load. How deep it is in the case has to be factored in.

Assuming same bullet weigh, same type bullet with similar bearing surface, and assuming seated to the same depth (Not OAL), then for the most part the answer is yes, but not guaranteed.

As always, back off and work back up.

 

[rsrocket1]

In general, if you seat a bullet deeper and reduce the powder to keep the same velocity, the pressure will be HIGHER than the original load.

You have a smaller volume and are trying to maintain the same velocity with the same bullet.

Consider a 150g bullet in a 30-06 case. Do you think you can get the same velocity by reducing the powder and putting it into a 30-30 case or a 30 carbine case? It would be great if that were possible, but it just doesn't work that way. That's the same principle taken to an extreme case.

Now, if you are 25% below max pressure with your 900 fps load with bullet "X", then you will probably be OK with bullet "Y" seated a few 0.01" deeper.

Pressure goes up faster non-linearly than velocity as you seat deeper so reducing the powder to keep the same velocity still results in a higher pressure.

 

[brickeyee]

The problem is that average pressure all the way down the bore determines velocity of the bullet, while peak pressure (in the first few inches of bullet travel) is what blows things up.

The two do not always track each other perfectly (average & peak).

 

[cfullgraf]

I log all bullet lengths as well as cartridge OALs so I can figure how much deeper in the case one bullet is over another load. How deep it is in the case has to be factored in.

Hmm, i had not thought of doing that before. Good idea. i have compared seating depths of old and new style bullets but never put some science behind it.

I guess that is why you get paid the "big buck".

 

[jim8115]

info

Bullet -180 gr truncated cone .625 long
Powder - 5.0 grains Power Pistol
OAL 1.085
Chrono'd at 905 FPS from a 4" XD

JIM