Pressure in a shotshell

[Paul Harm]

From a Accurate Powder Co. reloading handbook :
The fundamental difference between a shotshell and a typical center fire rifle cartridge is, that the efficiently of the shotshell is 100% dependent on the round itself. By this we mean that all the " resistive forces " must be generated within the confines of the round itself. No assistance is provided by the gun.
The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the the case. [ In the case of a CF rifle cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of leade/free-bore dimensions on the combustion process. [ ie. bullet/bore interface fit, bullet hardness, bearing surface, etc].
The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle cartridge is totally absent in a shotgun.
This means that the efficiency re ignition and the subsequent increase in pressure, is totally controlled by the integral configuration and assembly of the round itself. These constitute the main internal mass [ shot mass ], the internal volume [ wad design ], the dynamic collapse [ primary expansion ] of the internal volume [ collapsible section of the wad], plus the displacing of the internal assembly and the unfolding of the fold/crimp [ secondary/final expansion].
The way this COMBINATION interacts, will determine the efficiency Pressure impulse [ profile and time-base ] and the Peak-pressure vs Velocity [ P/V ]. The resistive force, presented by friction in a shotgun is negligible.
Crimping is certainly one of the most important aspects of the shotgun shell reloading process.
The influence of crimp on the ballistics is often ignored, and assumed to be of lesser importance than primers and wad make/design.
The fact is that the effect of Crimp-strength can totally overshadow the influence of the other components and parameters. This is controlled by the following : crimp depth, condition of the case, wad [ stiffness ], and wad tension.
With that said, from a article I once read, and don't have now, factory ammo has a crimp depth of around .050. When you read reloading data [ pressure] it's with a .050 crimp depth. Any reload with less depth will have less pressure, and deeper will have more pressure. If one likes less crimp in the hopes of increasing shell life, and loads at low pressures, bloopers could result. Or, if someone wanted to have a bit less pressure from given data, he could go to a .030 or .040 crimp depth. How much - I don't remember the numbers but, and this is a but, if only .020 was used the load would be reduced by 1000psi - but don't quote me. I do remember it quite a bit more than I would have thought. I'm sure the same applies to roll crimps. And that brings to mind something else - maybe that's why I could never get good reloads with brass cases unless I used black powder, and maybe that's why Charlie got good loads with nitro by gluing in the over shot card - it gave the resistance necessary for good combustion. Just some food for thought.

[Mark Ouellette]

Paul,

Darn good and informative post!

Mark

[Mark Garrett]

By Tom Armbrust:
To show what happens with various crimp depths, another test was run in the Hodgdon Ballistic Laboratory using one control reload and different crimp depths. Unless otherwise listed, the data in this text was created using a standard depth of 0.055, which is a bit short of 1/16. However, some manufacturing variations exist in which case handloaders are urged to use a factory equivalent crimp depth for that particular shotshell. The test reload was checked for pressure and velocity at 0.020 increments.

The test reload was assembled like this:

Shell: Winchester 12 GA 2-3/4” AA CF
Primer: Winchester 209 Primer
Power: 20.0 Grains of Hodgdon Clays
Wad: Winchester WAA12L
Shot: 7/8 oz of Lead Shot


The resulting ballistic data was as follows:
Crimp Depth Velocity (fps) Pressure
0.030" 1,308 f/s 9,300 PSI
0.050" 1,329 f/s 10,500 PSI
0.070" 1,351 f/s 11,900 PSI
0.090" 1,363 f/s 13,100 PSI



The importance of crimp depth, then, should be obvious: crimp depths to either the high or low side of normal will directly impact pressure/velocity results.

[Steve Havener]

It would be interesting to see if there is that much variation in pressures when using a roll crimp.

Does Tom address that?

[Frank Srebro]

This seems to discount the theory whereby a taller shot column - as with the same weight (mass) of bismuth vs. lead - will result in increased pressure due to more surface/friction of the shot against the bore, as the shot is accelerated. Just sayin.

[Mark Garrett]

This seems to discount the theory whereby a taller shot column - as with the same weight (mass) of bismuth vs. lead - will result in increased pressure due to more surface/friction of the shot against the bore, as the shot is accelerated. Just sayin.

How is that ?

[Pete Lester]

This seems to discount the theory whereby a taller shot column - as with the same weight (mass) of bismuth vs. lead - will result in increased pressure due to more surface/friction of the shot against the bore, as the shot is accelerated. Just sayin.

Where is the evidence of that? Do you believe that surface area of the shot column against the bore has no effect on pressure?

Case in point. Sherman Bell's data for the Short Ten shows a Fed Hull, Win 209 primer, 32gr of SR7625 with SP10 wad and some filler for both 1 1/4 ounce lead and 1 1/4 ounce of Bismuth. The lead load uses a folded crimp and generated 6700 psi. The Bismuth load using a Roll Crimp (which reduces pressure) generated 7100.

I suspect if that Bismuth load had used a folded crimp there would be somewhere around 1000 psi greater with the Bismuth.

[Mark Ouellette]

Where is the evidence of that? Do you believe that surface area of the shot column against the bore has no effect on pressure?

Pete,

I believe it would have an effect but compared to the other factors, especially with modern plastic wads, that effect would be negligible.

I also believe that all else being equal a slower burning powder will produce less felt recoil than would a faster burning powder. The results of tests of this theory are subjective depending on the individuate test participants.

I like theories but am forced to accept hard data.

Mark

[Paul Harm]

Mark G, thanks - the also was true when going with less crimp. I believe Mark O. in a previous post had some info about shot hardness effecting pressures. JMHO - lead shot will flatten a bit in the back part of the shot column, acting like the cushion part of the wad. This allows the expanding powder gas to expand more easily, meaning less pressure. Might be all wet with that theory, but there's some reason hard shot raises pressures. The roll crimp issue. Mark had also commented on deep roll crimps raising pressure and I would agree with him.

[Pete Lester]

Pete,

I believe it would have an effect but compared to the other factors, especially with modern plastic wads, that effect would be negligible.

I also believe that all else being equal a slower burning powder will produce less felt recoil than would a faster burning powder. The results of tests of this theory are subjective depending on the individuate test participants.

I like theories but am forced to accept hard data.

Mark

I think I gave a pretty good example of the effect.

[Mark Ouellette]

Paul,

My previous posts included such information as:

Young's Modulus stating "Everything is a spring".
http://parkerguns.org/forums/showthread.php?t=11293&highlight=young%27s+modulus

This is measured by the Birnell hardness of a substance.
http://parkerguns.org/forums/showthread.php?t=10717&highlight=brinell

Also, any opposition to the acceleration of the shot column will raise pressure. It has to because the powder is burning at a rate that increases under pressure.

[Mark Ouellette]

I think I gave a pretty good example of the effect.


Pete,

Sorry, but I reread your posts in this thread and did not see it. Please include a link or the information in this thread for consideration.

Mark

[Robin Lewis]

Case in point. Sherman Bell's data for the Short Ten shows a Fed Hull, Win 209 primer, 32gr of SR7625 with SP10 wad and some filler for both 1 1/4 ounce lead and 1 1/4 ounce of Bismuth. The lead load uses a folded crimp and generated 6700 psi. The Bismuth load using a Roll Crimp (which reduces pressure) generated 7100.



When loading shells, the shot is measured by volume and listed in ounces. I suspect the volume/ounce conversion is for lead. Do you think Sherman weighed or measured his Bismuth load? If he measured the shot and given Bismuth is less dense than lead, the Bismuth weight would be lighter than 1 1/4.... but generated higher psi. What would that indicate?

[Pete Lester]

Pete,

Sorry, but I reread your posts in this thread and did not see it. Please include a link or the information in this thread for consideration.

Mark

It is from Sherman Bell, check the last page of the Short Ten spreadsheet I put up this week.

[Pete Lester]

When loading shells, the shot is measured by volume and listed in ounces. I suspect the volume/ounce conversion is for lead. Do you think Sherman weighed or measured his Bismuth load? If he measured the shot and given Bismuth is less dense than lead, the Bismuth weight would be lighter than 1 1/4.... but generated higher psi. What would that indicate?

Since Bismuth loads are stated in weight and not volume I have always used an adjustable bar or a modifed mec bar that throws the desired of weight of Bismuth. I have never heard of anyone doing otherwise.

An equal weight of bismuth to lead is going to have a taller shot column.

PS. I am pretty certain Sherman weighed his Bismuth loads, the evidence is the difference in amount of filler wad needed for the same weight loadings vs. lead.

[Mark Garrett]

It is ( or should be ) standard procedure to weight all shot charges . Even with lead shot that is a different size or antimony will weigh different if thrown be volume . All shot bushings should be adjusted to throw your shot charges to the correct weight . Thought that was common knowledge.

[Frank Srebro]

From a Accurate Powder Co. reloading handbook :
The fundamental difference between a shotshell and a typical center fire rifle cartridge is, that the efficiently of the shotshell is 100% dependent on the round itself. By this we mean that all the " resistive forces " must be generated within the confines of the round itself. No assistance is provided by the gun.
The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the the case. [ In the case of a CF rifle cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of leade/free-bore dimensions on the combustion process. [ ie. bullet/bore interface fit, bullet hardness, bearing surface, etc].
The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle cartridge is totally absent in a shotgun.
This means that the efficiency re ignition and the subsequent increase in pressure, is totally controlled by the integral configuration and assembly of the round itself. These constitute the main internal mass [ shot mass ], the internal volume [ wad design ], the dynamic collapse [ primary expansion ] of the internal volume [ collapsible section of the wad], plus the displacing of the internal assembly and the unfolding of the fold/crimp [ secondary/final expansion].
The way this COMBINATION interacts, will determine the efficiency Pressure impulse [ profile and time-base ] and the Peak-pressure vs Velocity [ P/V ]. The resistive force, presented by friction in a shotgun is negligible.......

I was just quoting the reference attributed to Accurate Powder, who surely has objective modern data to substantiate what is written. It's copied in part above; please especially note the last sentence. I have a hard time understanding a substantial increase in bore friction because of a 1/4" or so longer shot column OF THE SAME WEIGHT riding in or mostly in a plastic wad.

I certainly subscribe to the advice that all should stick to published recipes for bismuth shot by reliable industry sources.

[Mark Garrett]

"These constitute the main internal mass [ shot mass ], the internal volume ."

1 oz of Bismuth has more internal volume and mass of 1oz of lead.

[Paul Harm]

I've always thought peak pressure was created within the shot shell , or at most, within the first couple of inches. With that said, Pete's comments about a higher shot column equals more friction, hence higher pressures. Pete also wrote in the Loading the Short 10 on page two :The difference betwen equal weights of Lead vs Bismuth is the height of the shot column and amount of surface area in contact with the bore. 29gr of 7625 is 29gr of 7625. With the lead load you generating 5900 PSI. Nice shot has the same size shot column as lead, but because it's harder than lead it generates more pressure as it has a harder time squeezing down through the forcing cone. This puts you up around 7400 psi with a fold crimp. Using a roll crimp will drop pressure, maybe 600, maybe 800 maybe even a 1000. Only testing will tell for sure. I would stay with the low pressure lead load and add 1500 psi. I can't imagine a sound Parker 10ga having an issue with an 8000 psi load. If you go too low you risk bloopers, especially in cold weather. -- I'm more than happy to agree with " friction in the barrel " meaning " friction in the forcing cone trying to squeeze ".

[Mark Ouellette]

Paul et all,

In a shotshell PEAK pressure is created in the chamber.

In a rifle peak pressure is created when the bullet is forced, essentially swagged, into the rifling.

When reviewing pressure curves notice that peak pressure of a shotshell occurs early in the curve as measured in time. Remember that the shot column is traveling rather slowly at peak pressure in comparison to muzzle velocity.

[John Campbell]

For the enjoyment of all, I post this older pressure reference:

[Mark Ouellette]

Thanks John!

The above chart is measured in inches from the breach. I'd love to have one of those test barrels!

For all 4 powders the peak pressure is reached no more than 1 and 1/2 inches from the breach. That distance is not far from the top of the powder charge as measured from the base of the shell. Hmmm, maybe that's why the chambers of barrels have much thicker walls than do the rest of the barrels... :whistle:

[Pete Lester]

Thanks John!

The above chart is measured in inches from the breach. I'd love to have one of those test barrels!

For all 4 powders the peak pressure is reached no more than 1 and 1/2 inches from the breach. That distance is not far from the top of the powder charge as measured from the base of the shell. Hmmm, maybe that's why the chambers of barrels have much thicker walls than do the rest of the barrels... :whistle:

"That distance is not far from the top of the powder charge as measured from the base of the shell". Which coorelates to the shot charge starting to enter the forcing cone and meeting resistance. Longer more gradual forcing cones have long been recommended for reduced felt recoil, lower pressure, better patterns. Along with longer forcing cones there is overboring.

"Over-boring reduces friction between the wad and the barrel as the wad travels from the chamber through the bore and out the muzzle. This reduction in friction is generally thought to reduce felt recoil and deformation of the shot pellets which results in improved pattern integrity and an increase center-pattern density."

http://www.doncurrie.com/over-boring-and-lengthening-forcing-cones

[Pete Lester]

"Friction being the highest of all the secondary force action on the wad/shot column as it moves down the barrel."

"My reasoning here is that the shot under acceleration of this force would try to spread out. Image trying to pile up shot on the top of your reloading bench the shot under just 1g of acceleration would rather spread out than stack."

http://mcb-homis.com/shotgunpressure/shotgunpressure.html

[John Campbell]

This is all grand discussion. However, it must always be kept in mind that currently fashionable "long forcing cones" are something to be avoided in good double guns. This includes Parkers. Why? Because to make the forcing cone long, the reamer removes metal where it is most needed. And in well-struck double gun barrels, that can result in dangerously thin wall thickness. Best to stick with standard forcing cones and a bit more "felt recoil."

[Paul Harm]

Pete, first problem is that Mr. MCB is an engineer :bowdown:. I don't believe I've ever read such a long drawn out article trying to explain perceived recoil. And in it he makes a couple of assumptions. :banghead:I fail to see how it relates to the discussion here about pressure increases with hard shot. If peak pressure is generated within the chamber [ and it is ], then friction between the wad and barrel is nill. Friction causing higher pressure would be an assumption unless someone has tested along the barrel. Then we would have higher pressures for quite a distance. JMHO

[Pete Lester]

Pete, first problem is that Mr. MCB is an engineer :bowdown:. I don't believe I've ever read such a long drawn out article trying to explain perceived recoil. And in it he makes a couple of assumptions. :banghead:I fail to see how it relates to the discussion here about pressure increases with hard shot. If peak pressure is generated within the chamber [ and it is ], then friction between the wad and barrel is nill. Friction causing higher pressure would be an assumption unless someone has tested along the barrel. Then we would have higher pressures for quite a distance. JMHO

The article points out friction is a factor in pressure. Hard shot has more difficulty hence resistance to flow from the chamber into the forcing cone, so what he is saying is more outward pressure is exerted on the sides of the barrel walls. Increase in friction, increase in pressure. Bismuth is harder than lead, steel even harder. Both have lighter mass than lead so the shot columns are taller. They both exert more pressure in the direction of the barrels and over a greater surface area as compared to equal weight of lead. The top of the shot column is jamming up at the beginning of the forcing cone holding back the rest or it in the chamber and that is where you have the highest pressure.

[Bruce Day]

So we have long discussions about perceived recoil, which by definition cannot be measured because if it could, we would have a formula to measure it. And we have a perfectly good, it seems, recoil formula which to my knowledge has stood unchanged for over 100 years.

With the elusive perceived recoil, one person may perceive it and another may not. It may be elusive as the Higg's bosun particle. Or it may depend on how may cups of coffee the shooter had.

[Paul Harm]

Bruce, the thread started out about what causes pressure in a shotgun shell and went to perceived recoil. Perhaps I misunderstood Pete's claim about friction in a barrel being a factor in pressure. I could believe hard shot doesn't compress as easily as lead when going through a forcing cone so higher pressures would be a result. I don't believe going down the barrel has anything to do with higher pressures. When writing instead of talking to someone face to face, sometimes it's hard to explain/understand exactly what is meant. I started the thread mainly to bring to every ones attention how important the crimp is and how it changes pressure.

[Bruce Day]

Paul, yes the thread drifted. And yes, crimp depth does affect pressure. However, when we are discussing Parkers, chamber pressure, in my opinion, should play little part unless a person is seeking to load up to maximum service pressure. We know what or about what those are for each gauge, but most of us, myself included, seldom shoot max pressure shells. The Parker max service load is more than we want or generally need to shoot, but its there if we need it unless the gun in question is loose, bored out, bad stock or otherwise in poor condition. I don't have guns like that but I know some do.

We have posters who somehow seize upon 5000psi as a magic number for 12ga chamber pressure. I don't know where that comes from, maybe light 2 " British guns, but not Parkers and its not a Parker number at all. Parker told us what to shoot with a range of acceptable loads which are still available today. For me, that's what I follow.

[Paul Harm]

Amen. I try to keep below 8000psi out of respect of 100+ year old wood stocks. Don't like to see cracks.

[Pete Lester]

Amen. I try to keep below 8000psi out of respect of 100+ year old wood stocks. Don't like to see cracks.

I always thought recoil affected stocks and pressure affects barrels.

[Bruce Day]

True. The recoil formula doesn't use pressure as a factor. However, we all know or should know that in general, the higher the pressure, the heavier and faster the load is ejected, which are recoil formula factors. Loaders can vary powders , primers and crimp depth to affect pressure . Chamber pressure and recoil are not fully separate although the formulas are. However, this subject matter is only important for knowledge purposes. For the general Parker shooter, he has a strong gun that can take substantial loads up to and including many of the so called high brass shells of today.....but not including some of these 1350 -1400 fps 1 3/8 oz loads that some manufacturers tell you that you need for killing pheasant.

[Paul Harm]

Guess I'm safe what ever way you want to measure - 3/4oz loads in a 12ga at 1170fps and most around 7000psi. Don't hunt, just shoot clays, so all that heavier shot loads aren't needed. My shoulder and stock are both safe.

[Steve Havener]

Sounds like a nice light load. Shooting a 28 gauge load in a 7 - 7 1/2 pound 12 gauge you should hardly notice the recoil.