doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure or velocity?

Does pressure by itself stress an action or is stress more related to the velocity of ejecta? In other words, if two loads have the same ejecta velocity but one develops 2,000 psi higher pressure do the two have the same stress on an action?

Pressure times area equals force. So it stands to reason that if you have two shotshells with equal area acting upon the breech, the one generating higher pressure will exert more force on the action... assuming equal velocities and equal mass of ejecta.

I'm in (rare) agreement with Keith. Pressure by itself does stress the action. If that were not the case, why would guns be proofed based on pressure standards?

Increased velocity also produces additional stress in the form of recoil. That stress, however, tends to work more on the stock than it does the action. If it were a significant issue for the action, then there would also be velocity standards built into the proof process.

It�s all about mathematics and physics. Force = mass x acceleration. Acceleration = Velocity(squared)/Radius. Pressure = mass x Velocity(squared)/Area ........ So, it appears pressure is related to velocity, but even more so to acceleration.

Remember also that if the two loads produce identical ballistics (same weight of ejecta @ same velocity) at different pressure levels the one with lower pressure will hold that pressure just a bit longer.

Time is the reason a lead crusher does not give a true PSI reading, always lower than recorded by a Piezo Crystal. The duration of the peak pressure in a gun chamber is so short it is nowhere close to that amount of static pressure.

I am in the camp that says pressure is the factor for determining barrel wall thickness to prevent a burst barrel. Ejecta weight (Shot, Wads & Powder charge) & Velocity determines the stress put on the action & also splits stocks.

The type of action we're concerned with here is presumably the actions of our break open double guns. But a good comparison and illustration of the very real effect that cartridge pressure exerts upon the action can be found in comparisons and calculations for bolt thrust in bolt action rifles. There is no question that bolt thrust and bolt lug shearing forces are greater in a cartridge with a larger rear inside surface area, assuming other things such as pressure are equal. Because of that, the bolt thrust generated by a .50 BMG is much greater than that of either a .222 Rem or a .30-06 even though the three cartridges are all scaled up or scaled down versions of the .30-06. And of course, the bolt thrust can be affected by the grip of a cartridge during firing in a dry chamber versus an oiled chamber. Here's an interesting link about the subject from Lilja Barrels:

http://riflebarrels.com/a-look-at-bolt-lug-strength/

And let's not forget that it is the force generated by cartridge pressure that operates the bolt in blowback actions. A Ruger 10/22 bolt weighs about .4 lb. to correctly counteract the roughly 1000 lbs. of rearward force exerted upon the bolt by a little .22 LR cartridge. If you built a blowback action for .50 BMG, it would require a bolt weighing about 50 lbs. to operate correctly and safely.

What is even more interesting to me is the fact that Larry can still see and read my posts even though he claims to IGNORE them!

Well let's see. Bullet weight times velocity of the ,50 BMG versus the .22 LR has a ratio of about 40:1. Max chamber pressure does not exceed a ratio of 3:1 for the Hi-Speed .22. Ratio of a 50 lb bolt versus a .4 lb bolt is a ratio of 125:1.

It very quickly becomes Obvious that max chamber pressure is not the Entire Equation.

The original question of course was concerning different loads in a given shotgun gauge. Internal shell dimensions would be constant "IF" the same hull was used for both. A quick look at loading manuals will show one can find loads which push 1� oz of shot up around 1300 fps at pressures down around 8K. We can also find 1 oz loads at around 1200 FPS @ 10K psi.

The heavier 1� oz load at the higher velocity will come nearer Cracking a weak Frame than will the lower velocity 1 oz load regardless of the different max pressures. Period.

The stress to the action is based on pressure --- pure and simple. Since the cartridge case can't possibly contain the burn pressure, the head of the case must be supported by the action face. The breech and bar carry the back thrust, and opening moments (force acting at a distance) are carried by the hook/pivot pin and the bolts and bytes. Pressure acts on recoil only as it causes acceleration of the ejecta and the resultant muzzle velocity.


Free rearward acceleration of the gun is identical to the acceleration of the ejecta, only opposite in direction. Since the gun has no non-trivial moving parts, recoil is identical to the ejecta mass and the muzzle velocity. Recoil is absorbed by the shooter via the stock. So, recoil is the factor for stock/shooter abuse.


DDA

On the first point, just for conversation and relating back to the original post. Let's say a six thousand psi shell and an eight thousand psi shell fire the same payload at the same muzzle velocity, that's a 33% higher pressure developed against the gun components, admittedly over a shorter duration. I can understand that the simple answer would be that any old gun can handle 8K psi, but could that percentage of 'stress' increase on an action matter?

And on the second point, if pressure affects acceleration, wouldn't the rate of acceleration be greater for the higher pressure shell? It would, again, be for a shorter duration, and in the end the area under the high and low pressure curves, the force, would be the same to result in the same muzzle velocity.

Though 'recoil' is identical for both shells that generate identical velocities, can the human nervous system detect that brief period of a higher rate of acceleration? Are some 'mild' loads able to 'pop' a shooter's shoulder and cheek sharper than others?

The only real way I would see to completely answer this question would be if an action could be fitted with a gauge of some sort to measure its deflection when fired. I do not have the capability of doing this nor do most others here.

Consider a few points though which have been determined over the years. Total barrel time from when the primer ignited until the load exits the barrel on most normal shotgun loads is on the order of .003 of a second. during that time the gun itself will have moved no more than about 3/8". The rest of the guns movement in recoil comes from the momentum built up while the load was traveling down the barrel plus a small percentage due to the gas exiting the muzzle.

As the pressure has peaked & is starting to fall before the load has cleared the chamber then it is at max pressure for an Extremely short time. As pressure is applied to the standing breech it takes a certain amount of time for deflection to occur. How much longer does 6K psi have to be applied to equal the stress on the action as 8K for a shorter duration.

Until shown absolute proof to the contrary I am still of the opinion that weight time velocity stresses the action & stock, PSI stresses the chamber walls.
1� oz @ 1300 FPS is about 20% higher than 1 1/8 oz @ 1200 FPS.
Within the confines of what will produce an acceptable load for a shotgun pressure itself does not enter into this equation.

As the OD of most shotgun barrels have more taper than does the chamber itself even as to the matter of stressing the chamber walls the higher pressure will occur where the walls are thicker than that of the lower pressure load so the difference is not as much as many suppose.

As of yet I have been shown no really Good & Valid reason for loading much below the 8K level. Most powders burn better at that level & you don't have to worry as much about the possibility of a squib if you shoot it in cold weather.

As to the recoil I guess I'm just not "Blessed" with the sensitivity of some. If for instance I load some shells with an ounce of shot to 1200 FPS I truly cannot tell you whether they are loaded with Red Dot, Green Dot or Unique, even though they have differing burn rates thus differing rates of acceleration. Up the velocity to 1290 FPS as are many of the factory promo loads which have to be able to function a semi-auto & yes I can Feel that.