Max Pressure (I don't get it)

Various stresses (and strains) are created in a firearm when it's fired. Chamber stress level (for example) is a function of pressure, diameter and wall thickness. The stress level created isn't supposed to exceed the elastic limit or yield strength of the steel.
 
Below is a chart from VarmintAl.com but it is computer generated guesstimate and base expansion seems to be high. And this base expansion varies with brass hardness, meaning military Lake City brass is the hardest brass and commercial Federal brass being the softest.

bzV8FFr.png

"1.
And as you can see at the link below Hodgdon's uses a lower base expansion amount than the chart above.

Simple Trick for Monitoring Pressure of Your Rifle Reloads
http://www.hodgdonreloading.com/reloading-education/tips-and-tricks/simple-trick-monitoring-pressure-your-rifle-reloads

The photo below is from accurateshooter.com and posted by a 1000 yard shooter. He loaded his Lapua brass until he had brass flow into the ejector and then backed off 1 to 2 grains of powder and called this max.

KtO65uH.jpg


And below is from the H.P. White Testing Laboratories.

"1.4 Failure of a gun assembly from internal pressure may be from either
of two (2) failure mechanisms.

1.4.1 The general perception is that those failures are the result
of a single exposure to a CATASTROPHIC PRESSURE level. This
may be an over simplification in that the strength of the
assembly may have been degraded by previous repeated exposures
to excessive, but lesser, levels of pressure whose cumulative
effect is to reduce the ultimate strength of the assembly.

1.4.2 Repeated exposure to pressures which exceed the elastic limit
of a material will continually reduce the ultimate strength of
the material until the ultimate strength is exceeded by a
relatively low pressure level causing fatigue failure."

Below hot loads with excessive bolt thrust caused the lug failure on this AR15 bolt. And a very good reason to keep the chamber and cartridge free of any oil or lube that doubles your bolt thrust.

PijdpgS.jpg


Below more excessive bolt thrust damage and over stressing the bolt lugs. And the early short magnums had a bolt thrust problem because the case body had so little surface area to grip the chamber walls. These rifles had to be strengthened to prevent early lug setback and its increased headspace.

cKCA1re.jpg


Another example, the original M193 cartridge for the M16 rifles was loaded to 55,000 psi. "BUT" the newer M885 round is loaded to 58,700 psi and is pounding our M4 carbines to death. And Quickload tells me my AR15 practice loads are below 50,000 psi and does not over stress my Lake City brass or my rifle.

The reason I posted the 65,000 psi 30-06 chart was arguments with fellow co-workers in the 1970s over which rifle was flatter shooting. I had the worlds best non-belted magnum the .270 Winchester and my buddies had 7mm magnums. Years later I learned a modern 30-06 could be loaded warmer and shoot just as flat as the .270 or 7mm magnum.

Bottom line, reloading is like playing Black Jack, stay light and beat the pressure dealer. ;)
 
Interesting -

"Repeated exposure to pressures which exceed the elastic limit
of a material will continually reduce the ultimate strength of
the material..."

Shear failure of the bolt lugs, seems to be the weakest link.:eek: Nice that the Mauser action employs a third safety lug using the bolt handle.
 
Slamfire said:
Interesting -

"Repeated exposure to pressures which exceed the elastic limit
of a material will continually reduce the ultimate strength of
the material..."

Shear failure of the bolt lugs, seems to be the weakest link.:eek: Nice that the Mauser action employs a third safety lug using the bolt handle.
Click to expand...

The Australians perfected the conversion of the No.1 Enfield in the late 1950s by using stronger steels. The problem was the cost of this conversion was over 50% of a new LA1A rifle and the project was dropped.

The conversion process was given to India and they made the Ishapore 2a1 adopted as a reserve rifle in 1963 by India. This is covered in Ian Skennerton's "Australian Enfield Variations" and explained how much the headspace increased using the standard No.1 action and 7.62 ammunition. The first .303 Enfield rifles were rated at 16.5 tons and progressed from 17.5 to 18.5 tons with the No.4 in .308/7.62 rated at 19.0 tons of chamber pressure.

On the flip side of this is the three loading limits for the 45-70 with two higher pressure loadings in the manuals for Marlin and Ruger rifles.
 
Last edited:
Estimated chamber pressure required to induce shear yielding of bolt lugs, for reference and illustrative purposes only (not for design) -

Shear Yield Stress SYS = 0.58 TYS (Tensile Yield Stress) [von Mises Yield Criterion]

For AISI 4140 which is typical firearm steel, TYS = 417.1 MPa = 60,495 psi (http://www.efunda.com/materials/alloys/alloy_steels/show_alloy.cfm?id=aisi_4140&prop=all&page_title=aisi%204140 )

Bolt thrust F = chamber pressure x case head cross-sectional area, P x A

Shear Stress, t = 0.5 PA / lug area, a' (2 lugs) = 0.5 PA / a'

FBD:

........................... <--- 1/2 PA (lug 1)
PA (bolt face) ---> |
........................... <--- 1/2 PA (lug 2)

Re-arranging, chamber pressure required to induce shear yield stress = 2 ta' / A

A = pi/4 x d^2, d = 0.532" for a typical magnum cartridge = 0.222 sq. in.
a' = say 0.40" x 0.50" = 0.20 sq. in.

P = 2 x 0.58 x 60,495 x 0.20 / 0.222 = 63,220 psi :eek:
 
Last edited:
Slamfire said:
Bolt thrust F = chamber pressure x case head cross-sectional area, P x A
Click to expand...

The bolt thrust figure above is dependent on the strength of the brass and chamber friction meaning its a ball park figure.

P.O. Ackley did a experiment with a 30-30 Winchester rifle and removed the locking bolt from the rifle. He removed all the oil from the chamber and remotely fired the 30-30 and nothing happened. With the case body firmly gripping the "dry" chamber walls and a chamber pressure of 38,000 cup or 43,000 psi the pressure was not great enough to cause the brass to stretch to meet the bolt face. The only thing abnormal about this fired 30-30 case was the primer protruded more than normal and only applied very little force to the bolt face.

Bottom line, a "DRY" cartridge case acts like a cars shock absorber and reduces bolt thrust and the dwell time the pressure is applied to the bolt face.

Meaning the figure you quoted above does not indicate the actual bolt thrust. Go back and read the section of the 1929 Textbook of Small Arms I posted about the base crusher system with oiled and and dry cartridges.

Do not take this the wrong way, but too many people quote this formula thinking a dry oil free chamber has the same bolt thrust as a oiled or lubed case. Meaning they think its OK to lube their cases and there is no increase in bolt thrust. This is why the computer generated figures in the VarmintAls .243 chart differ from Hodgdon's actual chamber pressures and base expansion.

Another example is Quickload computer software and chamber pressures and velocities. In order to get closer guesstimates with Quickload you need a chronograph and adjust the burn rate in Quickload until the velocities match your chronograph. Meaning you must tweak and adjust Quickload to get more accurate figures.

Bottom line, a few older British proof houses may still use the base crusher system and measure actual bolt thrust. And mathematical formulas for bolt thrust are very ball park at best.

Look at the link below from VarmintAls with chamber finish and friction "estimates" for a .243. and bolt thrust and dwell time. And remember they are computer generated guesstimates and not actual pressure pressure readings.

http://www.varmintal.com/a243z.htm
 
Last edited:
A bottleneck definitely slows the movement of propellant out of the case. It's the size of the pipe and the difficulty of finding the exit. OTOH, we love bottlenecks because that overbore capacity helps us keep a higher pressure on the bullet longer for higher velocities. It's not just the peak pressure that matters to the bullet. The average pressure and how rapidly the pressure builds are very important. There is also an effect on accuracy. When propellant burns/decomposes, the gas produced is very hot and in a bottlenecked cartridge hammers the unburned grains more effectively. The bottleneck helps the pressure in the case build faster and stay hotter longer until all the grains are consumed. That's a recipe for better/faster/more uniform ignition. That's a recipe for getting the bullet out of the barrel faster and more precisely timed from the release of the firing pin. This helps accuracy. Making whatever happens happen faster helps accuracy because it's more uniform. You could get most of the benefits of a bottleneck by making a really long cylindrical cartridge but that might be difficult to feed/handle. Bottlenecking was the right thing to do so Mauser and a bunch of others did it. Straight cases are still popular for large calibres and artillery where bottlenecks might be impractical.
 
Suther said:
This isn't a hard and fast rule. For instance, you can buy plenty of 45-70 ammo that is not safe to shoot in a trap door springfield. Some also question the safety of using modern 6.5x55 ammo in Norwegian Krag rifles. Or modern 30-06 in an M1 Garand. Im sure there are others, but you get the point.
Click to expand...

It was my understanding that the modern 30-06 in a garand issue had more to do with concerns about excessive gas port pressure when using most hunting ammunition, leading to accelerated wear on the gas system/damage to the op-rod, rather than the action not being strong enough.
 
dan belisle said:
270's were built on newer rifles with better steels than early 30-06's. Pressure isn't always just A mechanical reading. - dan
Click to expand...

I remember reading about 45-70 having 3 levels, old vs new firearms, and trapdoor firearms. I'm sure a new rifle in 30-06 would be a lot strobgwr than a 50 year old one. Or could be made stronger.
 
You must log in or register to reply here.
Back
Top Bottom