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How does the breech back-thrust of an average 12-guage shotgun load compare to that of some of our common centerfire cartridges?
 

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That depends on what condition the chamber is in, but the easy answer is-- Pressure is applied equally throughout the case, so how much 'back thrust' depends on the pressure and the area of the case head.
Center-fire rifles and shotguns are usually pressure tested using different methods and the results boiled down to 'psi', but notice that is pound PER SQUARE INCH, not pounds applied to the bolt or breech.

The friction of the expanded case against the chamber walls greatly affects the amount of thrust applied to the breech face. The Brits tested cartridges by greasing them and then measuring how much a lead slug was compressed by a moveable breech face. The results were written as "tons".

Suffice to say, the larger the gauge of the shotgun, generally the less breech thrust you have per square inch. The .410 bore shotgun is much higher pressure 'as it affects the gun' than a 10 ga. shooting the heaviest loads. (It's fairly common to see 're-enforced frame' double rifle actions used in .410 shotguns.)

Having said ALL that-- 12 ga operates at about 12kpsi. Centerfire rifles closer to 50kpsi. .410 bores close to 40kpsi.
 
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A 12 gauge produces about half the rearward force that a 30-06 does.
 

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It is not the case head size which effects case head thrust, it is the maximum interior diameter of the case which matters. And in many cartridges the case wall friction does not reduce case head thrust; Ackley pulled the wool over a lot of shooters’ eyes here. Any experienced reloader should know this because the case walls fail when there is excess headspace, resulting in a case head separation as the case head is forced back against the breach while the case walls stretch. Stronger brass withstands this effect - to a limited extent.

The calculated case head thrust for the 10 and 12 gauge and the .30-06 is below. These are based on standard SAAMI pressures in psi and an average maximum inside case diameter.

10 gauge - 5190 pounds
12 gauge - 4800 pounds
.410 gauge - 1780 pounds
.30-06 - 9970 pounds

It’s simple math, assuming I can operate a calculator....


PS - the .410 operates at 13,500 psi.

.
 

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The calculation is simple to figure out the force applied in one direction on the case, but you are ignoring a lot of other force vectors and friction coefficients. I read a research paper where tests were conducted with 30-30 win and 30-30 Ackley Improved with no bolt face on the back of the case. They were measuring the velocity of the cartridge as it expelled from the chamber. They found that the Ackley improved not only ejected at a reduced velocity, it actually many times did not even get all the way out of the chamber. Standard 30-30 ejected forcefully into the backstop every time.

Physics is Physics. It takes force to stretch the brass.
 

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Yes but for shotgun shells.... I doubt they 'stick' to the chamber near as much as brass. Plastic just doesn't have the tensile strength that brass does. Suspect that 'backthrust' on shotgun breechfaces is nearly the entire calculated amount.

A bit less for a brass cartridge that doesn't rupture. Easy enough to calculate the tensile strength of a ring of brass ;) so as to arrive at the max amount of 'stickiness' that the brass case can impart. Still.... if you blow a case head, that all goes out the window.

Anyways just some thoughts.
 

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I agree with you Mike on both counts. Just trying to make the point that the math is a a little more complex than

pie R squared P.
 

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Discussion Starter #8
I sense some disagreement here, as the 5190 (10g.) 4800 (12g.) 1780 (.410) figures don't square with the need to reinforce a .410's frame when compared to a larger gauge. And I am exactly trying to get a feel for the "how it affects the gun" notion as J Belk put it, as I think about which gauge shotgun barrel to order (20 or 12) for my T/C Encore. I know it can handle either, but I find it an interesting question nonetheless.
 

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A 12 gauge produces about half the reward force that a 30-06 does.
unclenick, I googled up "reward force" and came up with all sorts of different answers, none of which had anything to do with firearms. What are you referring to?

Yes but for shotgun shells....
Yeah those. I always wondered why hunting loads used "Hi Brass" hulls, when the pressures were the same as trap loads??
 

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I can't find my Lyman Shotshell Handbook that has the pressure figures I remember, but I do know the Model 42 Winchester is 'stronger' than a Model 12 (heat-treat mostly) due to the higher .410 pressures. BUT, it's a moot point. The TC Encore is plenty strong for much more pressure and intensity than any shotshell will offer.
The weak link is NOT the gun, it's the cartridge case. How well the case is supported by the gun determines if or how it'll hurt you should the case fail.

Low brass and High brass was a difference in how paper cases were made. With molded plastic hulls it makes no difference. The brass is just for the extractor to grab, now.
 
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unclenick, I googled up "reward force" and came up with all sorts of different answers, none of which had anything to do with firearms. What are you referring to?
Search for 'bolt thrust'.
 
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Think you're running the calculator right....it's pressure vs. square inches of case head. Shotguns seem mild pressured but have a pretty large diameter.



Considering the steel in a some12gr. breech faces...might just eventually get a compression-dent in the breech.....might get a failure at whatever does the locking or opening (which generally isn't as strong a lock up and off-center to the line of thrust).
 

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I sense some disagreement here
There is no disagreement, you should always listen to UncleNick's words. 😁



Having said ALL that-- 12 ga operates at about 12kpsi. Centerfire rifles closer to 50kpsi. .410 bores close to 40kpsi.
Cute but not quite.
The 410 runs about the same as the 12 gauge ranges. 12,500~13,500 psi MAP

 
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It is interesting to note that recoil force and 'back thrust' are not the same.
 

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unclenick, I googled up "reward force" and came up with all sorts of different answers, none of which had anything to do with firearms. What are you referring to?
I fat-fingered "rearward".
 
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IF you are considering a conversion of a shotgun....would help if we knew which shotgun. Some are actually tough enough to take it,,,,most aren't.
 

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I do not have a high school education but have run my own company for thirty years.
if you put ten psi in a 4x4 in box of welded alum no problem you put the same ten psi. in a alum fuel tank it will blow like a bomb pressure is the same for the area it is affecting 3 psi.. over 16 sq in.= 48lbs.x6 a cube = 288 psi.
Now make it 12 in. x 12= 144 x 6 = 864 psi. But this does not hold true for guns why ? my guess is you only get x amount of bang from a given amout of powder. The larger the chamber you ignite it in the less pressure you will get.
Bob
 

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Consider the top of the box being held on the edges by the welded seam. If you double the dimensions of the box, you quadruple the area of the top (to be acted on by the pressure) but have increased the length of the edge seam (holding it together) by only a factor of two.

Consider two rifles of different chamber volume, both operating at the same maximum pressure. Unlike your box analogy (which uses the same material for each box), the designer determines in advance the thickness, material, heat treatment, etc. to ensure safe operation of each.
 

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Let's not even get into the "forward force" exerted on the rifling! :)
 
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