High/er pressures: rifle vs shotgun

[Ds J]

Good evening Folks, I read somewhere that pressure works different between a rifle and a shotgun. Something along the lines that a rifle has a max load capability dictated by the combination of weight and friction, whereas a shotgun works "completely different".

How does it work?

[Treeman]

I so hope we get some answers here, this is a very interesting matter. I am sure 223 will have answers, the man is genius on some of the technical stuff.
Oom Drie, wat se' u ?

As an opening line:
I would imagine this to be very true, the dynamics are so different, rifles have bore resistance that is so different to   that of a shotgun, continuous sliding friction and rifling, bore over mass etc, so yes I think the rules are the same but derived for differing cause and affects. I am battling to say what I want to say. Shotguns go "bang" and its over, launched and then momentum takes over. A rifle is a much more prolonged matter with longer force and result times.
Here is another article sent to me by another knowledgeable man, Messor. It may explain some things better.
Messor by David Frank Allen, on Flickr
   

[oafpatroll]

Theres an article that I tried and failed to find that deals with pressures in shotguns. The author, a ballistician, performed experiments where he sought the failure point of a selection of shotguns by increasing powder charges till kaboom. The results where fascinating as they suggested that a modern shotgun can handle very substantial overcharges. My load development cycles with many buck and bird loads shown minimal increases in velocity for changes in charge weight if 10 percent or more. When I get back to a PC I'll look for the article and post it along with some pressure test results I had done for me by somchem.

[Tripodmvr]

A shotgun has a relatively unencumbered bore (no rifling) and a wad of shot that glides down the barrel in a plastic shotcup. The pressure needed is therefore relatively low.

A rifle bore is tighter and must push a very tight metallic plug (the bullet) through the rifling. The chamber pressure can be extremely severe, and is controlled by the thick walls of the firing chamber.

[Ds J]

If pressure is not such a problem in shotguns, why are shot cartridges relatively slow? And when is pressure too high ie when do explosions occur? It seems that most shot travels somewhere between 1200fps and 1700fps?

Is it just the weight of the loads, or something else?

Another thing I don't understand is the use of fast powders in shot shells. Why use a fast burning powder? It creates a faster pressure spike, and it does not make use of the long shotgun barrels?

[oafpatroll]

If pressure is not such a problem in shotguns, why are shot cartridges relatively slow? It seems that most shot travels somewhere between 1200fps and 1700fps?

Is it just the weight of the loads, or something else?

It's not that pressure isn't a problem in a shottie it's that for shot to work will it doesn't need to be travelling at light speed. The fact that it's sherical, relatively soft and generally not very big means that it can't go a very long way and diverges from the the centreline quickly. If you hotrod bird loads they tend to pattern really badly from shot getting deformed at launch and then veering off in all directions. The best patterning 28g 7.5 load I developed runs 1250fps. Shotguns are almost always built light and you most certainly can blow up even modern ones but it seems that the margin for error is more generous than in rifled firearms. 

This is the article I mentioned above.

https://www.shotgunlife.com/shotguns/tom-roster/important-information-about-shotshell-pressures.html

[Treeman]

If pressure is not such a problem in shotguns, why are shot cartridges relatively slow? It seems that most shot travels somewhere between 1200fps and 1700fps?

Is it just the weight of the loads, or something else?

It's not that pressure isn't a problem in a shottie it's that for shot to work will it doesn't need to be travelling at light speed. The fact that it's sherical, relatively soft and generally not very big means that it can't go a very long way and diverges from the the centreline quickly. If you hotrod bird loads they tend to pattern really badly from shot getting deformed at launch and then veering off in all directions. The best patterning 28g 7.5 load I developed runs 1250fps. Shotguns are almost always built light and you most certainly can blow up even modern ones but it seems that the margin for error is more generous than in rifled firearms. 

This is the article I mentioned above.

https://www.shotgunlife.com/shotguns/tom-roster/important-information-about-shotshell-pressures.html

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Paper cases and paper thin barrels do not lend themselves to high pressures, but like you said there is a limit to how you can accelerate the shot without it deforming or clumping. I do believe that once this was realised the door to light weight and less than rifle specifics was opened. Shotties were then made to shottie expected pressures.
Check this out as a matter of interest. Pressure - velocity - shot load ? 200 birds one shot>

  https://en.wikipedia.org/wiki/Punt_gun

https://www.youtube.com/watch?v=i-u9MmeXl8E

both interesting.

[Treeman]

Oh, check this out for a shottie
https://www.youtube.com/watch?v=i-u9MmeXl8E

[Tripodmvr]

A shotgun does not have rifling that causes pressures to peak faster, so the initial peak of faster propellants are necessary to make the shot cup seal off and keep the pressure down the barrel.

[Ds J]

I guess the next thing in shot loads  would be rubber covered lead shot, or something similar.

Rubber would reform after the initial pressure, and the lead would add to the weight.

OK, maybe I am mumbling too much.

The point is that there are good reasons for the design of current loads.

Thanks a lot!

[oafpatroll]

Steel and other hard metal shot is pretty much the next/current thing. Lead is being banned all over the place for environmental reasons. Loads based on that shot is hotter to impart more energy to the lighter shot to compensate for it running out of puff quicker. The volume of shot is also generally greater than for lead based loads of equivalent effect.

[Ds J]

Steel and other hard metal shot is pretty much the next/current thing. Lead is being banned all over the place for environmental reasons. Loads based on that shot is hotter to impart more energy to the lighter shot to compensate for it running out of puff quicker. The volume of shot is also generally greater than for lead based loads of equivalent effect.

Steel shot and bismuth seem to be the next popular loads. However, traditional barrels cannot always handle the "pressure" of the steel shot because steel does not compress and deform like lead. Hence, one should be careful especially in older shotguns.

[oafpatroll]

Steel shot and bismuth seem to be the next popular loads. However, traditional barrels cannot always handle the "pressure" of the steel shot because steel does not compress and deform like lead. Hence, one should be careful especially in older shotguns.

Absolutely. Many european guns have been specifically steel proofed since sometime in the 80s or 90s when the move away from lead started. I wouldn't consider shooting steel out of my bird gun for fear of buggering it.