FPS with CUP or PSI

Bill Daniel · January 22, 2010, 10:05 PM

A question for those in the know. I was looking at load data in Hornady's 2010 book and noticed that some powders run much higher pressures for the same or near same muzzle velocity. Does the lower pressure mean longer rifle life or does it matter since they are with in SAAMI limits? B-LC(2) 47 gr. 2754 fps CUP 50,200 Sierra 168 HPBT/ IMR 4895 45.4c gr 2758 fps. 58,000 PSI.
Thanks,
Bill Daniel

wncchester · January 22, 2010, 10:25 PM

CUP or PST, the pressure's the same, the effects are the some. Difference is the method of measuring the pressure.

**Unclenick** · January 22, 2010, 10:31 PM

Not the same above around 30,000 psi. Crusher measurements usually give lower numbers than psi measurement of the same load above that pressure. They're not interchangeable and have different SAAMI limits.

Faster powders reach their pressure peak before the bullet has gone as far down the barrel as it has when a slower powder is used. So, to make enough gas to achieve the same velocity, they have to cram it into a smaller volume behind the bullet. Hence, the higher peak pressure.

On the other hand, since that higher pressure gets more of the bullet accelerating done in an earlier portion of its trip down the tube, the total gas volume is still smaller than the same-velocity slow powder makes. Thus there is less pressure at the muzzle when the bullet exits, which can significantly reduce recoil. Note also that the faster powder, since it makes less total gas, has a lower charge weight. That saves money, at least, within the one brand.

The penalty is the higher pressure, which is why you can put enough more of the slower powder into a maximum load to get more velocity.

Yes, both safe. Best thing is to use the one that produces best accuracy in your gun.

January 22, 2010, 10:05 PM	#1
Bill Daniel Senior Member Join Date: February 10, 2000 Location: Bowling Green, KY. Posts: 518	FPS with CUP or PSI A question for those in the know. I was looking at load data in Hornady's 2010 book and noticed that some powders run much higher pressures for the same or near same muzzle velocity. Does the lower pressure mean longer rifle life or does it matter since they are with in SAAMI limits? B-LC(2) 47 gr. 2754 fps CUP 50,200 Sierra 168 HPBT/ IMR 4895 45.4c gr 2758 fps. 58,000 PSI. Thanks, Bill Daniel __________________ Prosecute criminals to the fullest extent of the law and their weapons will become harmless. "Let us speak courteously, deal fairly, and keep ourselves armed and ready." Theodore Roosevelt 1903

January 22, 2010, 10:31 PM	#3
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Not the same above around 30,000 psi. Crusher measurements usually give lower numbers than psi measurement of the same load above that pressure. They're not interchangeable and have different SAAMI limits. Faster powders reach their pressure peak before the bullet has gone as far down the barrel as it has when a slower powder is used. So, to make enough gas to achieve the same velocity, they have to cram it into a smaller volume behind the bullet. Hence, the higher peak pressure. On the other hand, since that higher pressure gets more of the bullet accelerating done in an earlier portion of its trip down the tube, the total gas volume is still smaller than the same-velocity slow powder makes. Thus there is less pressure at the muzzle when the bullet exits, which can significantly reduce recoil. Note also that the faster powder, since it makes less total gas, has a lower charge weight. That saves money, at least, within the one brand. The penalty is the higher pressure, which is why you can put enough more of the slower powder into a maximum load to get more velocity. Yes, both safe. Best thing is to use the one that produces best accuracy in your gun. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

January 22, 2010, 10:25 PM	#2
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	CUP or PST, the pressure's the same, the effects are the some. Difference is the method of measuring the pressure.