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

Only conversation Miller. In this example, I think time is a constant. From a standing start, both high and low pressure shells have identical muzzle velocities, so they must have spent the same amount of time in the barrel.

Both payloads are still accelerating at muzzle, because I believe PSI's have been shown at the muzzle. But, to keep the force, the area under plotted PSI curve, equal, the high pressure load would be accelerating at some lesser rate than the low pressure loads towards the end of its travel down the barrel. To keep the total force on the payload equivalent, the higher PSI's likely created a higher rate of acceleration right at that point it was measured, but the rate trailed off quicker than the lower PSI load.

As might also be the case in this example, the total force is also a constant, so the thought is, does applying equal forces in different ways, matter? I believe your point is related to the shorter duration of the exposure to the higher PSI, but the gun was subjected to the higher PSI none the less.

Miller, you answered your own questions and concerns when you brought up the lead crushers and piezoelectric transducers.

Typically, these are installed into a test barrel in a spot on the chamber where pressures will be greatest during the firing cycle. We all know that the lead crusher values give a lower reading, and that there is a percentage added to correct for that in order to equate it to the pressure the same load would generate if you were using the piezoelectric transducer. The example of primer flattening or flowing under extreme pressure is simply an exaggerated way of illustrating that there is a definite rearward force exerted by pressure. It doesn't matter if one load is 8000 psi and the other load is only 50 psi less at 7500. The higher pressure load will always have the higher rearward force, unless we can suspend Pascal's Law.

It also matters not that the pressure is not static, or that it is sustained for a very small fraction of a second more or less. The same correction factor for lead crusher testing is generally used across the range of cartridge pressures and with all different loads and powders. We all know that the lead crusher pellet or the piezoelectric transducer is going to tell us which load generates the higher pressure. And if that measurement were to be taken at the rear end of the cartridge rather than an inch or so from the breech on the chamber wall, then it will prove to us that the higher pressure cartridge stresses the action more. And as a machinist, you are certainly aware that engineers commonly add material to provide more strength to mechanisms which will undergo sudden shock loading. This is further recognition that the higher pressure load stresses an action more, all else being equal. And it takes us back to the very real fact that purpose built double rifle actions are built with more material in critical places than their shotgun counterparts. You won't find many gunsmiths doing double rifle conversions using Ithaca Flues or Tobin actions.

Consider that a static pressure vessel has no recoil. Gas pressure must create momentum to create recoil. Free recoil is completely described by conservation of momentum. Gas pressure creates momentum only in a flow situation. In the case of a gun, the flow is due to increasing volume within the barrel. Momentum can't be found from pressure. Pressure, in and of itself tells you nothing about a dynamic process.

It is easy to visualize the pressure of the powder gas pressing back on the breech and causing recoil. It is also wrong. If the barrel were plugged at the front of the chamber, firing would produce no recoil (if the gun held together). The barrels are locked onto the action and constitute a pressure vessel, but one that has expanding volume.

Momentum of the ejecta is matched by the rearward velocity and mass of the gun. Momentum is transmitted from the barrel/action to the stock and to the shooter. Pressure influences recoil only as it influences momentum of the ejecta.

Free recoil momentum of the gun an d ejecta total momentum are equal.

DDA

Don, are you proposing, with the above, that there is are no pressures exerted against the breech by the shell head? I think two debates are getting mixed up here. Of course, there is no recoil where there is no flow, but point is that, in a shotgun, there is flow, so there is recoil. I'm just not sure what point you are making here.



No, the two do not exert the same stress on the action. With two loads that are calculated to have exactly the same recoil energy, the one with the higher pressure will stress the action more. Not because of higher recoil. Again, the recoil is the same. But, because of the higher force exerted against the breech by the shell head in the brief moment that the pressure is still rising, and peaks. Regardless how long or short the duration of that event.

SRH

If I am not mistaken, I believe your original comment was that pressure could be related to acceleration. The change in momentum may be more of what is being felt in recoil.

If a static PSI is applied to a metal part, and it is deflected a measurable amount, will the same PSI, applied over a very short duration, deflect the same part to some measurably greater degree because the change in momentum of the deflection can't be stopped instantaneously? An example may be a stubborn bolt responding to a sharp hammer strike rather than a steady pull on a cheater bar.

Back to the original question. Anecdotally, we seem to know that guns can go off face. It seems to happen due to multiple exposures of short duration pressure exposures, and it seems to happen with less repetitions if the pressure is higher.

"A man convinced against his will,
is of the same opinion still." Stan

I never saw that one before. Good sense though...Geo

I understand that scientific methodology in deriving reproducible data and sustainable conclusions is vastly inferior to anecdotal data and opinion on this forum but here is something that may assist some of you.

http://www.claytargettesting.com/study2/Study2.4.pdf

Wonko;
Very good charts. "EVERYONE" notice in particular that last chart comparing Red Dot to PB. The Red Dot load showed about 12K psi with the PB load showing about 9K . Graph was only divided in 2K increments but this is extremely close. This thus gives about a 333% increase in pressure for the RD load. Now look at the Recoil Speed comparison, abot 6 for the RD & 5.8 for PB or an increase of about 3�% for the RD. Pressure increase is thus about 10 times that of the recoil increase.

I stand behind my hypothesis that the main determining factor RE spring or cracking a frame is shot weight, not pressure per se. You can throw in all the extreme things you so desire to which are not relevant to the original question of 6k vs 8K. Flattening a rifle primer at 60+K psi really doesn't have any bearing on this discussion.

Sure one can load a case full of detonating powder & blow a gun to smithereens without even moving the charge, but that is mere stupidity & again irrelevant.

In fairness to the original post, I added the numbers, only as a way to quantify possible pressure differences.

Doc has shared that piece before, and I think it's good, but it took your extrapolation to explain how it's even remotely related to stress on a gun. That's okay.

We all know that once we're travelling at a fixed velocity, we no longer feel the force it took to get there. Don't you think it would have been more relevant if the author could have measured acceleration, because both the shot weight and velocity are constants? Of course, we're going to get a constant velocity for the opposite reaction on the gun, aren't we?

The author explained that he arrived at the methods only because they were repeatable. Maybe, the graph in figure 4.4 is the most important of all. I would have preferred he didn't mix shells there, but it's all over the map and 'corrected'. Maybe, that points to inconsistencies in the shells and measuring capability, but if some shells attained their velocity in half the time, why were the shells that took twice the time to reach velocity 'corrected' back to the shorter time?

The thing I noticed about the Red Dot and PB chart is what we already know, it's documentation that different pressures can result in the same total opposite reaction on the gun for recoil purposes. But, it still leaves, good, bad or indifferent, one variable among constants, pressure.

Stan, I'm trying to be sure folks understand that pressure, in and of itself, does not cause recoil. The pressure acting on the hull base becomes a force which does, indeed, act on the barrel and action "pressure vessel," but does not cause recoil. The pressure acting on the base of the wad becomes an unbalanced force (since the wad and shot do not have enough resistance to balance the pressure force) causing the acceleration. The momentum gain will balance the pressure force on the wad base at every instant until muzzle exit.

DDA