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  #1  
Old 07-13-2019, 9:00 PM
steelholder steelholder is online now
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Default How to calculate psi and ft/lb?

Hey all I'm relatively new to reloading and I'm aware of how important pressures are to this hobby, I often see psi numbers and ft/lbs, for example 362 ft/lbs 18500 psi, is there an equation on how to get these based on powder charge, projectile weight, gun weight? Thanks guys.
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Old 07-13-2019, 9:13 PM
devster55 devster55 is offline
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I would think muzzle velocity would be your best bet. Quickloads could give you a close estimate as well I would think.
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Old 07-13-2019, 9:21 PM
steelholder steelholder is online now
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Quote:
Originally Posted by devster55 View Post
I would think muzzle velocity would be your best bet. Quickloads could give you a close estimate as well I would think.
Thanks howuch is that program? I'd like to know the formulas they use to get those numbers, I doubt it's that difficult, I could be wrong.
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Old 07-13-2019, 9:35 PM
saxman saxman is offline
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I believe you're looking for fps, not ft/lbs. One would measure velocity (fps) not torque (ft lbs) when reloading.
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Old 07-13-2019, 9:52 PM
steelholder steelholder is online now
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Quote:
Originally Posted by saxman View Post
I believe you're looking for fps, not ft/lbs. One would measure velocity (fps) not torque (ft lbs) when reloading.
That's what I meant, I understand what fps is but often you see a number like 462 ft/lbs 18000 psi, I just want to know what it means in relationship to loading and how it is calculated, here is a similar thread on XDtalk

https://www.xdtalk.com/threads/relat...recoil.221369/
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Old 07-13-2019, 9:59 PM
superdave50 superdave50 is online now
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It might help to add that you're talking about recoil. Most hand loaders only care about velocity and it's relation to accuracy.
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Old 07-13-2019, 10:29 PM
steelholder steelholder is online now
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Quote:
Originally Posted by superdave50 View Post
It might help to add that you're talking about recoil. Most hand loaders only care about velocity and it's relation to accuracy.
Im not talking about recoil, that's why I never wrote anything about it, the thread I posted may have mentioned it, I dont care for it, pressures are also very important, specially if loading hotter rounds..

Anyone know how those numbers are calculated?
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Old 07-13-2019, 10:37 PM
devster55 devster55 is offline
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Reading your posts this is exactly what you are looking for. Kinda pricey but well worth it.

https://www.neconos.com/details3.htm
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Old 07-13-2019, 10:37 PM
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Ft lbs is torque, which is how they're measuring recoil, so if you're asking about ft lbs, you're asking about recoil.

All those numbers are either indirectly measured or estimated. Velocity is much easier to measure than pressure, so that gets used the most. Is your question how to measure pressure?
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Old 07-13-2019, 10:40 PM
superdave50 superdave50 is online now
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Sorry, your link was pretty much all about recoil.

There's a few calculators out there.

http://reloadammo.com/footpound2.htm

"Energy = Weight times Velocity Squared divided by 450395 (fixed constant)"
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Old 07-13-2019, 10:57 PM
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Quote:
Originally Posted by saxman View Post
Ft lbs is torque, which is how they're measuring recoil, so if you're asking about ft lbs, you're asking about recoil.

All those numbers are either indirectly measured or estimated. Velocity is much easier to measure than pressure, so that gets used the most. Is your question how to measure pressure?
Yes, in a way, Im aware of the mathatical equation for pressure but some rounds are considered "high pressure" and I think standard pressure for 45 acp is 21000 psi, how would one know the psi for a hotter load like +p or super. I hope I'm making sense here.
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Old 07-13-2019, 10:59 PM
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Quote:
Originally Posted by superdave50 View Post
Sorry, your link was pretty much all about recoil.

There's a few calculators out there.

http://reloadammo.com/footpound2.htm

"Energy = Weight times Velocity Squared divided by 450395 (fixed constant)"
Thanks this is partially what I was looking for, anything like this for psi? I'll try Google but please post link if you guys know of one
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Old 07-14-2019, 2:46 AM
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Quote:
Originally Posted by superdave50 View Post
Sorry, your link was pretty much all about recoil.

There's a few calculators out there.

http://reloadammo.com/footpound2.htm

"Energy = Weight times Velocity Squared divided by 450395 (fixed constant)"
Quote:
Originally Posted by steelholder View Post
Thanks this is partially what I was looking for, anything like this for psi? I'll try Google but please post link if you guys know of one
Ft lbs, is not just a measurement of "torque" or "recoil".

As superdave showed. It is simply a measurement of energy potential. Also used, and shown in some reloading manuals as a "lethality" indicator. Or "how hard" a given bullet at a given velocity HITS the target at given distances.

PSI, on the other hand, as noted, is a "pressure/force" indicator. At lower levels like water or air pressure. Simple to use a gauge. At firearms pressure level. Not so simple, and impossible to measure with a mechanical dial gauge.

Also far to many variables to be able to reduce the measurements at that level. That happen in thousandths of a second to any equation.

In early firearms developement "copper crushers" were used. Where a tiny copper ball of a given diameter was subjected to the pressure until it deformed. Then the deformation was measured and used to calculate the pressure required to cause the deformation. That was known as "CUP" [copper units of pressure]. Explained in this link.

https://en.wikipedia.org/wiki/Copper_units_of_pressure

Now it is done "electronically". Using either "strain gauges" or "piezoelectric cells". Which measure the massive force levels in thousandths of a second.

Electricity depending on the medium it is moving in. Travels at about 280,000,000 meters per second. In a 12 gage copper wire. Much faster than any bullet.

And also far beyond the capability of any hobbiest reloader. Unless you have tens of thousands of dollars to spend setting up your own ballistics lab.
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Old 07-14-2019, 10:22 AM
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Default How to calculate psi and ft/lb?

$700 will get you the software and hardware you need to test pressure in a gun.

https://www.shootingsoftware.com/mm5...tegory_Code=PT

I am sure there are others out there.

You then need to scratch a small spot on your gun barrel to glue the sensors (strain gauge) to. A Thompson center contender with the barrel in the caliber of your choice is perfect for the job that way you donít destroy your gun. Just a tiny spot on the contender barrel behind the rear sight itís almost not visible

Mike


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Old 07-14-2019, 10:47 AM
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All you digital twits need a Powley psi computer. Lets see if any old time reloaders here or a buncha kids.
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Old 07-14-2019, 6:22 PM
rsrocket1 rsrocket1 is offline
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Quote:
Originally Posted by saxman View Post
Ft lbs is torque, which is how they're measuring recoil, so if you're asking about ft lbs, you're asking about recoil.

All those numbers are either indirectly measured or estimated. Velocity is much easier to measure than pressure, so that gets used the most. Is your question how to measure pressure?
Ft-lbs is used to measure torque but it's also used to measure energy (Force times distance).

In ballistic calculations, many people try to relate muzzle energy to how powerful a bullet is in terms of "hitting power". This is one way to compare the potential of a bullet to cause damage to a target but it's got limitations.

To calculate muzzle energy, you use the formula 1/2*m*V^2
Where m= mass of bullet
V = muzzle velocity V^2 = Velocity * Velocity

This is the kinetic energy of the bullet at the muzzle.

We usually measure the mass of the bullet in grains and the velocity in feet per second. These do not instantly translate into foot-pounds, but the conversion isn't too difficult if you know these two numbers.

To convert bullet weight and muzzle velocity into foot-pounds, you use the 1/2*m*V^2 formula but you have to convert the bullet weight into "slugs" where 1 slug = 225,218 grains.

This webpage makes it easier to translate bullet weight and muzzle velocity into foot-pounds

http://www.larrywillis.com/bullet-energy.html

For comparison:
in the UK, pellet guns are limited to 10 foot-pounds before they require licenses to own.

A typical 22LR that shoots a 40g bullet 1100 fps has a muzzle energy of
107 foot-pounds

A 120g 9mm bullet with a MV of 1100 fps has 322 foot-pounds of energy

A 120g .357 bullet with a MV of 1400 fps has 522 foot-pounds of energy

A 55g .223 bullet at 3000 fps has 1098 ft-lbs of energy

A 150g .308 bullet with 2760 fps has 2567 ft-lbs of energy

Energy is simply the potential of a bullet to cause damage. If a 3000 fps .223 bullet passes clean through an animal and does not expand, tumble or fragment, very little energy is deposited into the animal.

It's easy to measure muzzle velocity. You just need a chronograph.

What you feel with your hands or against your shoulder (recoil) is more related to bullet momentum which is the mass of the bullet times the muzzle velocity (plus burned powder mass times velocity)

This is a calculation of exterior ballistics.

Calculating pressure within the cartridge or gun is a matter of interior ballistics.

This is more complicated and best left to reading load data manuals although you can estimate maximum chamber pressure with some software (Quickload) or measure the pressure with a device like Pressure Trace although you shouldn't bother with this unless you really know what you're doing and want to go really deep into interior ballistics.

If you follow published load data, work up your loads and stay below the maximums, you'll certainly be good to go for a long time. With more research and experience loading, you can go as deep as you wish. No need to go this deep when you're just beginning.
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