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

Some can, yes...............without a doubt I can. And it has nothing to do with pain at all. It feels like it is the duration of the recoil ......... "pop" or "push". When I switch loads sometimes is when I notice it. After the first few rounds that difference fades and is no longer noticeable to me.................until I switch again.

As to the pressure having any effect on the standing breech, I have changed my mind on the matter. I now believe it does. To draw a comparison with a rifle cartridge, one of the signs of too hot a load (too high chamber pressure) is flattening of the primer. The only way possible for the primer to flatten like it does is for the head of the case to move against the bolt, be it a rotary or falling block, or the standing breech of a double rifle. Granted, we are talking about much higher pressures in a rifle cartridge, but that doesn't change the principle. The primer is flattened because pressure is exerted on the case/shell head, from the inside of course, forcing it against the breech face, whatever type it may be. I can find no other possible explanation for flattened primers in hot loads than that. The pressure tries to blow the primer out of the pocket, but cannot because it has met an "immovable" object, so it deforms it into any space available. I understand steel flexes, so please don't take me to task for using the term immovable.

SRH

Joe,

Let me give a simple illustration. Load a 30/06 with a 150 gr. bullet, 57.5 grs IMR 4350 and it should produce 2930 fps with 57,300 psi according to Hodgdon's data. If one would use Bullseye to match the same velocity with the same weight bullet, the pressure would be quite excessive and could well result in serious damage or injury.

It would be the pressure that caused the damage, not the velocity. I suppose one could say if the desired velocity is reached while the bullet is still in the chamber, then velocity is the cause. Post hoc ergo propter hoc.

The book ISBN 0-9645598-4-6 Understanding Firearm Ballistics Basic to Advanced Ballistics Simplified, Illustrated & Explained deals with stuff like this. It is so exciting to read I doubt I have read more than one chapter since I bought it many, many years ago.

Note that the original question was between a 6k max pressure load versus an 8K pressure one in a shotgun. Both would presumably be loaded with a suitable powder & both are "SAFE" pressures.

BullsEye is "NOT" a suitable powder for loading a 150 grain bullet to a velocity of 2930 FPS in a .30-06, PERIOD! Try it & yes you will most likely Wreck your Rifle.

I believe we do need to keep our answers relative to the question being asked. Try & load a 1� oz load in your 12 gauge to 1300+ FPS using Red Dot or similar fast burning powder & you will also likely wreck your shotgun. That However was not the question being asked.

As to the original question which stated the Same shot load was pushed to the same velocity with one load producing 6K psi & the other 8k psi. To this point no one has stated any "Facts" which causes me to change my answer.
The difference in the strain on the action will be so small (Even though not Necessarily Exact) as to be inconsequential.

Primer flattening in a high pressure rifle load is entirely out of the scope of this question. When the primer ignites in a rifle cartridge it actually tends to shove the case forward to the extent of its head space. Pressure from the powder after it ignites pushes the case back against the breech reseating the primer. When loading low pressure cast bullet loads in rimless cases the primer thrust can set the shoulder of the case back & the pressure may not be enough to set it back. It is always advisable to keep such cases "ONLY" for further low pressure use & not to reload a full power load as it would have the same effect as excess headspace, "BUT" this is not the question we were discussing here.

Miller, Pascal's Law is a fact I can provide to prove that there will be more force or stress exerted upon the action of a gun firing two loads of equal velocity and equal mass of ejecta. And THAT was the question in the original post:



Pascal's Law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container. The container is the shell or cartridge, of course, and the fluid is the gasses formed from the rapid burning of the powder. The pressure does not just act perpendicular to the axis of the bore. The same pressure that requires thicker barrel walls at the chamber is also pushing equally on the area at the inside of the rear end of the shell, and pushing with equal force upon the end of the wad containing the shot. The same force that pushes the wad and shot charge down the barrel also pushes equally in every direction within the shell. And to briefly return to the example of primer flattening in a rifle cartridge, this would still occur with excess pressure even if there was zero headspace. In fact, with really hot loads, we see this rearward pressure actually causing primer metal to flow back into the firing pin hole.

The difference in stress or force upon the shotgun action from the higher pressure load may not be all that great, but it is undeniably greater in the higher pressure load. And that was the original question.

What you are not looking at here is the Time factor. Which will flex the standing breech the most 6K psi for X Microseconds or 8K psi for Y Microseconds.
Your formula while certainly true does not truly address this for such a short duration of time.

Explain by Pascal's Law why a lead crusher does not give the same pressure reading as a Piezoelectric crystal, the same pressure is applied to both.

Let's turn this around & say that if I have two loads both producing 8K PSI. One load uses 1� oz shot @ 1330 FPS while the other uses 1 oz shot @ 1130 FPS. As I understand what you are saying from Pascal's formula both would stress the action identically. Both should thus have Identical Recoil, I think we all know this is simply not true. Another "Law" states for Every Action there is an Opposite & Equal Reaction". The Reaction of 1� oz of shot 200 FPS faster is simply greater than the 1 oz load even though pressures are identical. "IF" on the other hand the payloads & velocities are identical then the Reactions are identical even though the pressures are different. The total picture simply has to be looked at.

Sticking to the original question, all that really matters is that if either your lead crusher or piezoelectric transducer was placed at the breech end of the shell, the higher pressure load would still crush the lead pellet more. And the piezoelectric transducer would still show more force transmitted from the higher pressure load.

We can play with all kinds of what-if's and hair splitting over milliseconds of peak pressures But the simple question was about which load of equal velocity and equal mass of ejecta would stress the action more. Going back to rifles, this is why a Winchester High Wall is suitable for higher pressure cartridges than a Low Wall. And it is why firearms designers have chosen good steel to make shotgun actions versus many other materials that would be cheaper and easier to machine. It is also why bolsters are added to reinforce the frame of double rifles.

"BUT", you spent nearly as much wordage telling me why my example was irrelevant as I did stating it.

The point of the example is to point out the principle that, as keith stated above, the pressure inside the shell/hull acts in ALL directions............... including against the inside of the case head, driving it back against the breech. This reaction is why higher pressures do stress the breech more than lower pressures. The flattening of primers was entirely relevant, and I heartily disagree with your claim that it is not. It PROVES that the case head is driven rearward with great force into the breech. And that, was why I used it as an example.

I'm done with this one.

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

SRH

Shouldn't it be the 'total' reaction, or force, is identical, or equal but opposite. The reason the total picture in simple terms isn't always what we're looking at, is the same reason someone might prefer a performance car accelerating to 59mph in 4 seconds, then accelerating an additional 1 mph for the next 16 more seconds. While an econobox might steadily accelerate, but require the entire 20 seconds to reach 60 mph. Is there a requirement to simply conclude that they both weighed about the same, and both achieved the same velocity, so they have to be looked at as simply the same?

Back to the gun analogy, which car is going to get torn up sooner if either only had their one path to get to 60mph over 20 seconds. Or, if it's a vintage car, just because it can be done, should it? Are there times, for the sake of preservation, that it may be worth taking a different path, not for everyone or all situations, but is it worth considering?

6K psi = 75% of 8K psi. I don't have any absolutes but would suspect that the primer flowing is up in the area of 60K psi or 750% of the 8K. This is why I said it was irrelevant to the question asked.

As I said in a prior post the question will not truly be answered until someone performs an actual test in which a strain gage is placed across the gap & precise readings taken.

What I am convinced that many are forgetting the shortness of time the pressure is actually applied. All of the examples which are being used would be completely true "IF" a static load was applied. "IF" we plugged the end of the chamber & pumped up pressures of 6K & 8K then the action would be stressed 75% as much with the 6K pressure as the 8K. This is not the case though. Either the 6K or the 8K pressure in the shotshell will not deflect the standing breech of a break open shotgun to the same extent as would this static pressure.

When this breaks through then perhaps it will be recognised what I am saying. working on the premise that the charge will travel down a 30" barrel in about .0003 seconds with the paek pressure occupying no more than about �" in the chamber or 1/60th of the total barrel length then assume the time involved at peak pressure is around .00005 seconds. Assume this to be for the 8K load. Then the flatter peak curve of the the 6K load would take around .000066 seconds. What then will be the deflection to the standing breech with 8K being applied for .00005 seconds versus 6K being applied for .000066 seconds. I honestly don't have a definitive answer but my opinion is different than many have.

"IF" you fail to take into consideration that both Force & Time are involved as a static pressure level is not reached you simply have not looked at the entire equation. Things are a lot more involved than just saying that 6K pressure will stress the action only 75% of what 8K would. It might be nice if it were that simple, but its NOT.