Pressure in a shotshell
 

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.

Paul,

Darn good and informative post!

Mark

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.

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

Does Tom address that?

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 ?

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.

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

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.

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