Ruger Forum banner

1 - 7 of 7 Posts

·
Registered
Joined
·
298 Posts
Discussion Starter · #1 ·
All the information that Iowegan provided in my HP-38/9mm thread got me thinking about the pressures created by different powders and their burn rates.

Am I correct in assuming that when the powder manufacture refers to pressure, be it PSI or CUP, the powder manufacture is referring to the pressure created in the pistol or rifle chamber (where the cartridge is inserted before it is fired) rather than the barrel (even though the chamber is part of the barrel)?

Javier
 

·
Registered
Joined
·
6,346 Posts
The pressure is measured at the chamber; that's where the pressure is the highest.

Jim
 

·
Registered
Joined
·
1,300 Posts
I believe over the years we've shortened the real term; "chamber pressure", down to just "pressure"...
 

·
Retired Moderator & Gunsmith
Joined
·
17,044 Posts
rojasj, Pressure tests for both the crusher system and the piezo transducer system are done in a special test barrel .... not an actual gun. Both testing procedures use a piston at a right angle to the body of the case .... where one end is touching the case and the other end is either compressing a copper pellet (crusher method) or is impinging on a piezo transducer. SAAMI pressure tests are measured at "mid-body" of the case whereas NATO and CIP (European equivalent of SAAMI) test chamber pressure at the case mouth. It's interesting to note ... after pressure peaks and the bullet is traveling down the bore, pressure drops off really fast. For a 9mm 5" barrel, chamber pressure with 115gr factory ammo will be about 32k psi after 1" of bullet travel but will drop to 8k psi at the muzzle .... 4 1/4" of bullet travel, (5" barrel) pressure drops about 75%.

Chronograph testing is done with actual guns. Most reloading manuals will indicate what brand and model gun (and barrel length) was used for the tests. Yes, there are devices available to test chamber pressure on the gun itself but these devices don't work well on revolver cylinders or semi-auto pistols where a slide covers the chamber and/or barrel. They do work pretty good on bolt action rifles where you have open access to the chamber/barrel.

With multiple piezo transducers located on a barrel and coupled to a computer, the pressure "trace" can be plotted from the time the primer flashes until the bullet exits the muzzle. For humans, this is a very fast period of time ... just .000375 seconds but for modern electronics, it's a snail's pace. The trace will show a sharp rise in chamber pressure until pressure peaks. Pressure will then ramp down pretty fast. Keep in mind ... a tiny bullet is being pushed by 32,000 lb of pressure at the beginning and is still being pushed by 8,000 lb of pressure when the bullet exits the muzzle.

I just happen to have a pressure trace from QuickLOAD (38 Spec +P) using W-231 powder in my Photo Bucket. The Red line indicates chamber pressure whereas the Blue line indicates velocity. Some things to note: peak pressure occurs at 1.3" then drops to 3800 psi when it exits a 6" barrel. The powder is totally burned up at 3.2". Velocity accelerates really fast in the first 3" of bullet travel then continues to accelerate but at a much slower pace as the bullet travels down the bore. An actual pressure trace from piezo transducers looks much the same as the computer simulated trace from QuickLOAD .... except it won't be as smooth. There will be notable changes when the primer flashes and again where a bullet crosses the B/C gap. Hope this was more help than confusion.
 

·
Registered
Joined
·
6,346 Posts
One quick point. Chamber pressure can be measured on an actual rifle (or pistol) using a strain gauge attached to the outside of the chamber. This has to be calibrated by using test rounds that produce a known chamber pressure and is only accurate to +/- 10% or so. The calibration process is a pain, but once it has been done, the strain gauge transducers can be left on the barrel and used repeatedly (modern transducers look like a piece of foil and won't interfere with the use of the rifle). So, if you've got a lot of money you don't have any use for....


Jim
 
1 - 7 of 7 Posts
Top