All things being equal, is it fair to say pressure is directly related to veloctiy?

[41 Colt]

If you had a number of .38spl reloads, all loaded using the same brass, primers, bullet weight and design and dimensions (OAL, seating depth etc.), and they all chronied to 770fps, but were loaded with different charges of different powders, is it fair to say they would all be roughly the same PSI/CUP?

Also, for a given powder and charge, if you change to a lighter bullet, will you get faster FPS and the same or less pressure?

And how would you compare these two .38spl loads in terms of pressure:

158gr LRN 770fps vs 148gr WC 690fps

 

[rnbra-shooter]

If you had a number of .38spl reloads, all loaded using the same brass, primers, bullet weight and design and dimensions (OAL, seating depth etc.), and they all chronied to 770fps, but were loaded with different charges of different powders, is it fair to say they would all be roughly the same PSI/CUP?

No.

It would be fair to say that they had the same AVERAGE PRESSURE.

But the different powders type/weight combinations will almost certainly be giving a different PEAK PRESSURE. And it is peak pressure that is measured when talking about "the pressure" that a load develops.

Also, for a given powder and charge, if you change to a lighter bullet, will you get faster FPS and the same or less pressure?

Less pressure.

If you go to a somewhat lighter bullet, the speed will be higher. But if you go to a much lighter bullet, the speed could be lower.

And how would you compare these two .38spl loads in terms of pressure:

158gr LRN 770fps vs 148gr WC 690fps

Without saying which powder was used, there is not enough information to get an answer.

 

[1899]

With respect to the first question, I don't think the pressures are neccessarily the same or similar. The shape of the pressure curve will vary with burn rate and the chemistry of the powder. One powder may produce a lower peak pressure, but over a longer period of time, thereby equalling a different powder that has a higher peak pressure, but maintains that peak over a shorter time.

For example, according to IMR/Hodgdon data:
with a 158gr Cast LSWC
3.1gr of Clays gives 871 fps at 15,100 CUP
9.0gr of H4227 gives 887 fps at 12,100 CUP

In that example you have a higher velocity with lower pressure.

 

[easyrider604]

A slow powder, say 800X, can deliver the same velocity as a fast powder like 700X.

However, 800X will produce lower pressures and therefore easier on the gun.

 

[jethunter]

No.

It would be fair to say that they had the same AVERAGE PRESSURE.

But the different powders type/weight combinations will almost certainly be giving a different PEAK PRESSURE. And it is peak pressure that is measured when talking about "the pressure" that a load develops.



Less pressure.

If you go to a somewhat lighter bullet, the speed will be higher. But if you go to a much lighter bullet, the speed could be lower.




Without saying which powder was used, there is not enough information to get an answer.

I agree completely with this. ^^

I have played with Quickload and simulated pressure spiking from 12,000 psi to 30,000 psi just by seating the bullet .2" deeper in the case in small capacity pistol cartridges, using Unique powder. But velocity increased less than 50 fps even though peak pressure more than doubled. In this example velocity alone would be a piss-poor indicator of peak chamber pressure.

 

[41 Colt]

I think my questions have been answered, thanks.