I've searched the internet for a formula to help determine the chamber pressure of a 9 mm round and have come up empty. I'm not one to "push the envelope", but I would like to know the approximate pressures. I got a chronograph as a gift and I am interested in finding the perfect 9mm recipe for my Beretta 92FS.
Calculating chamber pressures for 9mm
- Thread starter robstr
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AmbulanceDriver
Member
rob, unfortunately I don't really think that there's an easy formula for converting muzzle velocity to chamber pressure. You can get an estimate of the average chamber pressure (possibly) assuming you know the kind of friction your barrel/bullet combination experiences, but trying to figure out the peak pressure is impossible from muzzle velocity alone. This is why reloading books are so important. You can compare your known muzzle velocities from your cronograph to the mv's in the book and estimate your peak pressure from the data they provide. This pressure data is obtained using some pretty specialized equipment that the average reloader is not likely to have..
The Bushmaster
Member
The perfect recipe for a Beretta 92FS? The one that is within the listed safe load data and is accurate in your handgun...Most loading manuals will give you approximate chamber pressures with their listings. Most of these are from test barrels and will not tell you what you will have, but will be close. Too many variables determine chamber pressures to have a working mathamatical formula to compute it. Lyman 48th Edition load manual is one of your best bets.
No such thing as a formula for chamber pressure. That's why manufacturers use complex equipment to measure pressure. The best use for your chrono is working up accurate loads. Don't be surprised if your loads are fatser or slower than what's listed in the reloading manual.
Clark
Member
CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.
There is allot of safety margin in 9mm pressures.
Lawyered up low risk corporations are selling +P+ 9mm ammo.
Take the barrel out of the pistol.
Put a round in the chamber.
With a needle, scribe the outline of the feed ramp on the case.
Measure the distance from the base of the case to the scribed line.
Most 9mm pistol barrels seem to have a feed ramp that is .190" or shorter.
The web of the case is .160" thick.
That leaves .030" of thin unsupported case wall.
To get enough pressure to get a guppy belly with so little area is hard to do.
Measure the barrel chamber wall thickness. Most are .1" or more.
To get enough pressure to split that chamber wall is hard to do.
Thin wall hoop stress:
S = [Pressure][inside diameter]/[2 thickness]
http://www.varmintal.com/arock.htm
http://www.kel-tec.com/p11.html
RC47 is the hardness, so 208ksi for stress in the steel
Chamber Pressure = [Stress][2][thickness]/[inside diameter]
P = 208k [2 ] .113" /.39" = 121,000 psi chamber pressure
That is not exactly right, because there is an open end of the hoop, which makes the stress higher, and there is brass to stretch first, which makes the stress lower. And Lame's thick wall formula is more accurate, but you get the idea, the barrel can take more than the brass.
The max load for 9mm 147 gr Power Pistol is 5.7 gr
http://www.alliantpowder.com/reload...gtypeid=1&weight=147&shellid=1014&bulletid=23
I have personally been shooting 158 gr .357" XTP with 11 gr Power Pistol in my Kel-Tec P11 9mm pistol.
Shooting a magazine full of that makes my right hand hurt for hours.
What does it all mean?
If I still have all of my fingers, you can stop worrying about the pressure in your 9mm book loads.
There is allot of safety margin in 9mm pressures.
Lawyered up low risk corporations are selling +P+ 9mm ammo.
Take the barrel out of the pistol.
Put a round in the chamber.
With a needle, scribe the outline of the feed ramp on the case.
Measure the distance from the base of the case to the scribed line.
Most 9mm pistol barrels seem to have a feed ramp that is .190" or shorter.
The web of the case is .160" thick.
That leaves .030" of thin unsupported case wall.
To get enough pressure to get a guppy belly with so little area is hard to do.
Measure the barrel chamber wall thickness. Most are .1" or more.
To get enough pressure to split that chamber wall is hard to do.
Thin wall hoop stress:
S = [Pressure][inside diameter]/[2 thickness]
http://www.varmintal.com/arock.htm
http://www.kel-tec.com/p11.html
RC47 is the hardness, so 208ksi for stress in the steel
Chamber Pressure = [Stress][2][thickness]/[inside diameter]
P = 208k [2 ] .113" /.39" = 121,000 psi chamber pressure
That is not exactly right, because there is an open end of the hoop, which makes the stress higher, and there is brass to stretch first, which makes the stress lower. And Lame's thick wall formula is more accurate, but you get the idea, the barrel can take more than the brass.
The max load for 9mm 147 gr Power Pistol is 5.7 gr
http://www.alliantpowder.com/reload...gtypeid=1&weight=147&shellid=1014&bulletid=23
I have personally been shooting 158 gr .357" XTP with 11 gr Power Pistol in my Kel-Tec P11 9mm pistol.
Shooting a magazine full of that makes my right hand hurt for hours.
What does it all mean?
If I still have all of my fingers, you can stop worrying about the pressure in your 9mm book loads.
Owen
Moderator Emeritus
It can be done, but you need to know a lot more information than you have easily available to you. First, you need to be comfortable with partial differential equations. Then you need to know the burn rate of the powder (which is itself a partial differential equation), the exact geometry of the barrel and chamber, the exact geometry of the cartridge case, the elasticity of the cartridge case, the slop in the headspace, the force required to unseat the bullet from the case, the moment of inertia of the projectile, the engraving force, the coefficient of friction between the bore and the bullet, and so on and so forth. This is all made easier because the 9mm is only slightly tapered, so the nozzle effects in a bottleneck case aren't particularly apparent with 9mm.
After doing all of this you should be within 10 to 15% of the actual maximum pressure. However, this information will do nothing to indicate the acccuracy of the cartridge, or if the gun will actually feed or cycle the round.
I went through the calculations once as an exercise to prove my dorkiness. I will never do it again without a significant financial carrot dangling in front of me.
I suggest picking up a loadbook, finding the bullet that you want to use, picking a midrange load for that bullet, seating the bullet to the max cartridge length, and seeing if it works. If it works, then you're done. Go shooting.
After doing all of this you should be within 10 to 15% of the actual maximum pressure. However, this information will do nothing to indicate the acccuracy of the cartridge, or if the gun will actually feed or cycle the round.
I went through the calculations once as an exercise to prove my dorkiness. I will never do it again without a significant financial carrot dangling in front of me.
I suggest picking up a loadbook, finding the bullet that you want to use, picking a midrange load for that bullet, seating the bullet to the max cartridge length, and seeing if it works. If it works, then you're done. Go shooting.
Liberty4Ever
Member
Holy poop on a stick! Did you chrono those loads?
The P-11 is approved for +P, but only for intermittent use because the frame flexes and can crack with repeated use of overpressure loads. The loads you're shooting are probably +P+. If you want to take that risk, fine, but I'd hope you were very up front with anyone else who might want to shoot your reloads (regardless of the weapon), and I'd hope you'd be decent enough not to take advantage of Kel-Tec's very generous lifetime warranty if you cracked a frame shooting such high power 147 gr rounds in a tiny concealed carry pistol that really isnt very well suited to 147 gr ammo of the normal variety, much less extreme overpressure loads.
I'm developing some +P loads for the 9mm SUB-2000 that use slower powder and as much as I can cram into a case to take better advantage of the 16" barrel. The slower powder reduces the felt recoil too. I suspect that even though the velocities are higher, the slower powder results in a lower peak pressure, but without an instumented test barrel, I can't really know for sure. No visible signs of overpressure yet in the brass, and I'm ordering some new +P brass today before taking the tests any farther.
I believe that safety limits are there for a reason, and I'm generally not comfortable cutting into the margin of safety for more performance. I might try it myself if I was very curious, but I'd probably fire the weapon from behind a barrier, I wouldn't make a habit of it, and I wouldn't request any warranty repair if the damage was caused by my own negligence or intentional abuse.
Clark
Member
Liberty 4 Ever,
I put the warning at the top and an executive summary at the bottom.
I have blown that pistol to pieces a couple times with AA#5 loads 6 years ago.
The case bulge and case head failure pictures have been all over the High Road for years.
I paid Kel-Tec for my replacement extractor, extractor pin, extractor spring, hold open, and magazine follower. The barrel has never been hurt.
A different manufacturer stated in the manual that their .45acp pistol was NOT rated for +P. When they read that I was shooting double loads, they sent me spare parts for free.
I think that is it possible for me to post my experimental data. It helps some with a point of reference: Just how big are the safety margins?
Father Knows Best
Member
Dude, you succeeded!owen said:
In all fairness, if I understood the math, I probably would have tried to do it, too.
The bottom line here is that it is practically impossible to "calculate" chamber pressures. Powder companies derive their published pressure data (in psi or CUP) by using test fixtures, not calculations. Home reloaders like us have to rely on published load data, and the traditional signs of overpressure in cartridges.
COPPER Under Pressure...Not...Chamber Under Pressure
The acronym C.U.P. stands for COPPER Under Pressure (and I think some call it Copper Units of Pressure). The companies have varying hardnesses of copper billets, and they deform at varying degrees of pressure. Try a search using the proper wording.
Link:
http://yarchive.net/gun/ammo/copper_units_of_pressure.html
http://en.wikipedia.org/wiki/Copper_units_of_pressure
Doc2005
The acronym C.U.P. stands for COPPER Under Pressure (and I think some call it Copper Units of Pressure). The companies have varying hardnesses of copper billets, and they deform at varying degrees of pressure. Try a search using the proper wording.
Link:
http://yarchive.net/gun/ammo/copper_units_of_pressure.html
http://en.wikipedia.org/wiki/Copper_units_of_pressure
Doc2005
Father Knows Best
Member
deleted as unnecessary
Liberty4Ever
Member
Clark wrote:
I do think it's interesting, even though I'd never make or shoot such drastic overloads. But I was sincere in my question. Did you chrono any of those loads? Just curious.
Modern firearms have wide safety margins. They should. Otherwise, guns explode, and that's not good for anyone. I still want those wide safety margins. Safety is good. Ten fingers are good. Two eyes are good.
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