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Formula for estimating pressure? (psi)

16K views 17 replies 13 participants last post by  dukalmighty 
#1 ·
Is there such a thing, and what data is needed to figure it? I'm sure my daughter could help me with the mathematics if I knew the formula and what to plug into it.

Thanks!
 
#5 ·
Cartridge case pressure.

farronwolf said:
One question though, why are you wanting to be able to measure the CUP pressure in the first place, are you manufacturing firearm parts, or simply reloading?
Just curiousity, for reloading. I have one load that I worked up to (4.6g HP38 under a 158g Lasercast RNFP). In the Hodgdon Data it's the max +P load but it's for a 158g XTP. It makes sense to me that the pressure should be slightly lower with the Lasercast bullet, given that it's not jacketed and I just wanted some way to confirm that this is true. I've no intentions to exceed that load, as it shoots well, there is no difficulty with ejection, and I've not noticed any bulging or anything. It's just another way to achieve peace of mind and back up my "real world" testing.

ETA: oh yeah...and because I still enjoy learning new things.:smile:
 
#4 ·
The best formula I know of is to look at the reloading manuals and see what they say a loads pressure is supposed to be. Work from the manual and inspect the cases like hp said.

It takes a lot of equipment to measure CUP pressure on rounds, something that is left generally to cartridge manufacturers and reloading manufacturers, not the average reloader.

One question though, why are you wanting to be able to measure the CUP pressure in the first place, are you manufacturing firearm parts, or simply reloading?
 
#7 ·
Thanks msgt/ret!...this is going to take some figuring out...
 
#10 ·
The best way to get a feel for pressure is to use the QuickLoad software program which was developed for that exact purpose. It does internal ballistics simulations to predict what happens from powder ignition till the bullet leaves the muzzle. The SW isn't for everybody, but if you want a way to figure it out, that is the way.

There isn't any simple formulla - just the powder data alone is a mess. QuickLoad contains a powder library, also a bullet library and cartridge library. It has features to screen for powders that might be suitable for the cartridge of interest. It comes with a really good manual which is well worth reading.

There is a good review of the SW here: QuickLOAD Review & User's Guide

I've been using it for 5 years now. It's been a really good resource for understanding and predicting cartridge behavior.

Fitch
 
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#12 ·
Thanks Fitch, I'll look into this.

claude clay said:
pressure....re-loading

look at the spent primer

is the edge still rounded? is ok
the case slide out easy fromcylinderender, no brass splits
--pblow backowback on the case can be cause of low pressure; the case did not expand to press against the chamber walls

is the edge of the primer flat to the brass? you are high pressure...possibly a +p or ++p
no blow by or any will be located in one area indicating the cut of the chamber not being perfect

primer flat and overflowing the hole, pofiring pine firingpin has punctured the face of the primer--
possibly the case is stuck and when you get it, it shows stress marks/crack
with a revolver, it may not revolve cause the primer has pushed out

--you are in dangerous territory. don't shoot any more of those loads..
Thanks for the very practical information claude :smile:. What I am after specifically though is numbers, to use in addition to all of the visual checks that you suggest. It's just a thing with me, I like numbers.
 
#11 ·
pressure....re-loading

look at the spent primer

is the edge still rounded? is ok
the case slide out easy from the cylinder, no brass splits
--blow back on the case can be cause of low pressure; the case did not expand to press against the chamber walls

is the edge of the primer flat to the brass? you are high pressure...possibly a +p or ++p
no blow by or some, will be located in one area indicating the cut of the chamber not being perfect or a thin area of the brass. check other spent casings for smiliar marks in the same location.

primer flat and overflowing the hole, perhaps the firingpin has punctured the face of the primer--
possibly the case is stuck and when you get it, it shows stress marks/crack
with a revolver, it may not revolve cause the primer has pushed out

--you are in dangerous territory. don't shoot any more of those loads..
 
#13 ·
Like the Retired Master Sergeant, I have a set of the old, cardboard fabricated Powley Computers - for loads, velocities and pressure prediction. Neat items.

There are some devices to measure pressure directly from your firearm. Not cheap or as simple as one might like, but available.

Ballistics Software, Chronographs & Pressure Instruments For Shooters

Being one such place. I have no connection with them, other than purchasing some of their hard and soft ware.
 
#14 ·
Yes, the bigger the bulge, the higher the pressure.
 
#16 ·
burn rates

If this thread isn't dead... can anyone come up with a burn rate for a powder? Any powder really. I looked at the formula that was created by Powley but that assumes that the psi is a constant unless I have read it wrong. I am currently trying to write a program that does something very similar to the slide calculator mentioned for my engineering class but I have been running into roadblocks everywhere trying to calculate a crude muzzle velocity given the bullet weight, caliber, grain. Is this impossible? Any and all advice is greatly appreciated!!
 
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