doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure vs recoil in old guns

Let me put this more bluntly.

Assume I have one shotgun. It's old. It's in good shape, but it's 100 years old.

Walking along one day i come to a fork in the road.

On the right fork is a greasy guy with slicked-back hair and a truckload of ammo that reads:
calculated recoil=almost zilch
Chamber pressure=SAAMI Max
This guy tells me that if I take his fork in the road, I can have his truckload of ammo to shoot my gun to my hearts content.
WHAT WILL HAPPEN TO MY GUN IF i TAKE THIS ROAD AND SHOOT MY GUN TO MY HEARTS CONTENT??

On the left fork, is another guy--dressed in somewhat unfashionably old clothes and smoking a pipe--with a wagonload of ammo that reads:
calculated recoil=hope you have a heavy gun with a thick recoil pad
chamber pressure="It's a miracle, but nice consistent ignition in any weather, 500psi (yes, five HUNDRED psi)"
The guy tells me that if I take the left fork I can take the entire wagonload of ammo to shoot my gun to my hearts content.
WHAT WILL HAPPEN TO MY GUN IF I TAKE THIS ROAD AND SHOOT MY GUN TO MY HEARTS CONTENT??

Assume my stock wood is just fine and the recoil is a non-issue ot the shooter, I'm only concerned about the gun or any issues caused by the gun. Which road is better for my gun? Which road will cause my gun to "shoot loose" faster assuming it's well-maintained in all other respects? Why? What are the best books or references that might help one learn about this in order to make my own decisions?

Pressure blows out the barrels almost instantaneously. Your gun would be beat to pieces more slowly.
The pressure to pop the barrels is high instantaneous psi, whereas recoil is in momentum units.
Maybe rocketman can clearly explain radial pressure versus time and overlay it on a recoil vs time graph. One happens before the other, in a perpendicular direction to the shotgun holder.

I have seen the figures for chamber pressure of the lowly .22LR rimfire, but do not have them handy now. I seem to recall they were at least approaching the 20K psi range. This is far above the the max allowable SAAMI pressures for even a modern 3½" 12ga Mag, yet I do not recall anyone ever mentioning "Shooting a gun Loose" from using one of the .22LR insert bbls fitted to a shotgun bbl, nor splitting a stock. My original post may not have been "Technically" correct, but in laymans terms I do believe it to be 100% correct. To figure the strength of a tube the important consideraation is pressure. To calculate frame strength, stock strength etc the important consideration is "Back Thrust" what ever terms you want to call it. Most looseness in break open shotguns is the result of frictional wear from improper maintenance & lubrication.
While certainly pressure is a contributing factor the "Back Thrust" is a combination of the pressure over the area it pushes against & the factor seldom mentioned of time.
I can fire a 2½" .410 with ½oz of shot @ 1200 fps & attain 11K psi. I can fire a 12ga 3" with 1½oz shot @ 1315 fps with 10.7K psi both using Hercules/Alliant 2400 powder. Assume both of these loads fired from guns having same stocking, same strength of frame etc, etc & I do not by the most elaborate of imagination believe I really have to ask anyone here which load would put the most strain on that gun. If you want to determine what load will come the nearest to "Shooting a Gun Loose" calculating the recoil of its load will be far more meaningful than will its chamber pressure. Incidently both of the above mentioned example loads were taken from an Alliant Manual & used Rem Hulls & Rem Wads. I hand-picked them for "Close" pressures but with any difference being for higher in the smaller .410.
I could just as easily point to 2 3/4" 12ga loads of 1oz/1200fps vs 1¼oz/1330fps; both @ an identical 9,500 psi. the lighter load simply "Will Not" "Shoot the Gun Loose" as quickly as will the heavier one even though both have an identical chamber pressure.
"I Rest My Case"
Do understand of course, the chamber pressue must be within the limits of what the chamber walls can take. I would certainly not recommend shooting an olg gun with thin chamber walls with ammo having pressures approaching SAAMI standards regsrdless of how light the recoil. If we look to extremes with a fast enough powder bursting pressures could be reached while producing almost no recoil as the pressure could rise so fast virtually no movement of the shot would have taken place while the pressure escaped out that big Gaping hole in the chamber side. The stock would of course be intact.

Gotta be careful where you attach that ONE string. Might get some sort of pinwheel rotation about a single center and a barrel dent, or heaven forbid, a stock ding. And gun sure as hell wouldn't accelerate at the same rate (relative "steepness" of plot for you math sophisticates--you know who you are) or to a maximum velocity equal to that of the ejecta. Mass x Distance / Time still balances both sides of Mr. Equal Sign.

jack

This is confusing...

Say you have two shells one is high pressure and one is low pressure both are 1 & 1/4oz. 12 ga. loads.

How is the higher pressure shell going to kick less than a low pressure shell ?

...and the engineer said "you can't do that. That violates the laws of physics." The politician says "that's what we're here for, to change the law".

This certainly sounds like recoil--not pressure--is by far the bigger gremlin, at least when it comes to wear and tear on the action of a gun. Is that fair to say?

I am curious about the second part of the observation though.



Given that I shoot very mild-recoiling loads (mostly 7/8oz <1200fps), what would be the difference to a gun between shooting a lot of 7000psi loads vs the same quantity of 11000psi loads? How would this likely manifest itself in the gun? I ask because it seems a matter of degree--my assumption is that at some point the pressure becomes "too much", and that's the working pressure the gun was designed to handle...but I assume it won't burst if I exceed that by just a little ("a little"=11k psi is still less than the gun was proofed at), so is any damamge likely to happen at all, is it likely to be cumulative, how will it manifest itself? Will this gun shooting 11000psi loads "shoot loose" faster than if I were using the 7000psi loads? Will any additional damage (i.e. the damage above what you would get from the lower pressure loads) be largely confined to the barrels, or would it encompass all of the action parts as well?

When we speak of pressure here we are speaking "Only" of the peak maximum pressure. The higher pressure shell is going to "Kick Less" only if it has a lower velocity. Again turning to my trusty Alliant manual I find these loads; A 3¼DE load with 1¼oz shot at 1220fps having a max pressure of 10,300psi with 23 grains of powder X. Using same hull, same wad & same primer I can change to powder Y & give that same 1¼oz of shot a 3½DE, 1275 fps, using using 34grains of this slower powder @ a max pressure of only 8,600psi. Since this load is scheiving 55 fps more velocity it would Kick Harder even though having a peak pressure 1700psi lower. "If" however you had a pressure gun which measured the "Entire" pressure curve, you would find that even though it has a lower peak, its average pressure under the entire curve would be higher. That lower peak will be offset by a higher pressure down the remaining length of the bbl.
The answer to your question is really very basic balistics. The recoil is based on the total wt of what goes down the bbl; shot, wads & powder & the velocity they attain. The pressure is based on the burn rate of the powder vs the wt it lifts (pushes down the bbl). A faster powder can reach a higher pressure while attaining a lower velocity & thus a lower recoil.
In the above example the 55fps extra velocity would more than offset the 11 grains wt of extra powder to be pushed out the bbl in the form of gases. This is about a 4.5% velocity increase with only a 1.8% wt increase. Pressure does not enter into the formula for calculating recoil at all, only motion & wt.

David;
When figuring the strengths of steel there are several factors which are involved. The two which holds the most interest to us here are Ultimate tensil & yield tensil, some times referred to as the elastic limit. The ultimate is of course the point at which the steel will break or in our case burst. The yield is the point at which it will accquire a permanent bulge. When pressure is applied inside the chamber it will "Swell" slightly, but upon removal of the pressure will return to original size. pon the pressure reaching the yield strength it will immediately take a sudden increase in its "Swell" & will not return to original size upon removal of the pressure, thus bulge has resulted. If the pressure is high enough it will of course continue to swell until it reaches the bursting point. Now lets assume that by knowing the yield strength of the steel we could then calculate that the pressure required to stress a given chamber to its yield point was 12K psi. "Theoritically" then we could fire a million rounds @ 11K psi with no detrimental effect. However another factor rears its Ugly Head, this may not be the correct technical term, but we have to consider the "Fatique Factor". This is simply saying if we go too close to the limit for too many times eventually the steel will just "Get Tired" & fail even though we never actually reached its failure point. This is of course one of the major reasons why service pressure is only a fraction of proof pressure. The best advise I can give is to simply keep ones loads within the service pressure for which the gun was designed.

I agree with all of your statements concerning recoil and pressure and their effects on the gun, Miller. And, as far as "calculating recoil" goes, with the above statement, too. Calculations, however accurate, do not totally agree with what I perceive the recoil to feel like. And, to take it one step further, and to parlay it into a partial answer to David's question as to how it affects his gun, I think burn rate of powder and the resulting "sharper" pressure spikes resulting from faster burning powders' use (read promotional loads here), also deliver a "sharper" blow to my shoulder, but more importantly to the head of the wood stocked gun. Even though the calculated ft.lbs. of recoil may read the same on paper.

A black powder load pushing 1 1/4 oz. of lead at 1100 fps will not feel nearly as "punchy" to me as a very fast burning smokeless load of the same weight and velocity out of the self same gun. I know you must have felt that difference as well. I believe that "sharpness" of recoil, i.e., the same calculated recoil delivered in a shorter time span will deliver more punishment to the wood at the stock head. Agree?

Truth be known though, that difference may be lost on a well inletted stock head and a piece of very dense and dry wood. If not lost totally, the differences in damage to the wood may, or may not, be incalculable.