takedown bolt shotgun and sub-caliber inserts

KotKotofeich

KotKotofeich

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How does one calculate bolt thrust correctly?

It has been said that break action shotguns are not suitable for sub-caliber inserts in rifle calibers - even .223 or 7.62x39, much less 308, because bolt thrust would destroy them.

Shotguns made out of bolt action military rifle would obviously have much stronger actions, but putting the sub-caliber insert is pretty much impossible.

I've seen a couple take-down shotguns made from 1871 mausers and subcaliber inserts can be put in ok. One could even make extractors for sub-calibers - they are very easy to swap.

My question is how to calculate bolt thrust this action can take? Just go off the energy of the original 43mauser cartridge? Or does the conversion to take-down make the action weaker? If so, how much?
 
You can calculate backthrust using the area of the inside of the head of the cartridge case and the maximum pressure of the cartridge. Something like the .223 has a very small area upon which the gas pressure is applied, so backthrust is less than a larger case operating at the same pressure.
Firing pin diameter and fit are also to be considered. The tip-up shotguns offered with rifle barrels will have small diameter firing pins closely fit to the hole in the breechface.
You could calculate the backthrust of an 11mm Mauser cartridge using area and pressure as described above. Measure the area of the single locking abutment, and you could calculate the load applied to the locking system. The M71 Mauser is not considered to be a particularly strong action. And has a large firing pin tip.
I assume that these takedown shotgun conversions have barrels that thread in.
I doubt that the M71 is any stronger than a modern bolt action shotgun.
 
You do know that a lot of shotgun actions have been used for calibers like .577 NE or 8x57 or 30-06 or .308 etc.....
Many fine Merkel or German double shotguns guns, especially in the 20 ga frame size are the basis of many double rifles ((9.3x74R, 30 Blaser, etc.).
I have several BRNO o/u actions that have a 12 ga combined with 5.6x52R, or 7x57 or .308 or replaceable with o/u 12 ga bbls. I use a sub caliber in many of them as well, including 45-70.
Those are all merely shotgun actions.
 
farshot said:
You do know that a lot of shotgun actions have been used for calibers like .577 NE or 8x57 or 30-06 or .308 etc.....
Many fine Merkel or German double shotguns guns, especially in the 20 ga frame size are the basis of many double rifles ((9.3x74R, 30 Blaser, etc.).
I have several BRNO o/u actions that have a 12 ga combined with 5.6x52R, or 7x57 or .308 or replaceable with o/u 12 ga bbls. I use a sub caliber in many of them as well, including 45-70.
Those are all merely shotgun actions.
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I suppose those are designed to be able to handle the rifle cartridges, but that is a good point.

The statement about break actions not being suitable to rifle cartridge inserts comes from MCA sports - they tried, did not have good results. I'd trust them on that subject - they make a lot of those inserts.
 
What were their "not good results"?
Receiver damage? Bbl bursting?
Chiappa makes a kit of sub-adapters to go with their folding survival shotgun, in 20 and 12 ga. They advertise that the adapters may be used in any SAAMI spec break open shotgun.
I find this interesting and would like to know more.
 
tiriaq said:
You can calculate backthrust using the area of the inside of the head of the cartridge case and the maximum pressure of the cartridge. Something like the .223 has a very small area upon which the gas pressure is applied, so backthrust is less than a larger case operating at the same pressure.
Click to expand...

Did you mean to say more 'more' ? Force over a smaller area > Same force over a larger area?


tiriaq said:
Firing pin diameter and fit are also to be considered. The tip-up shotguns offered with rifle barrels will have small diameter firing pins closely fit to the hole in the breechface.
You could calculate the backthrust of an 11mm Mauser cartridge using area and pressure as described above. Measure the area of the single locking abutment, and you could calculate the load applied to the locking system. The M71 Mauser is not considered to be a particularly strong action. And has a large firing pin tip.
I assume that these takedown shotgun conversions have barrels that thread in.
I doubt that the M71 is any stronger than a modern bolt action shotgun.
Click to expand...

Would you (or anyone) have the max pressure for 43mauser? Or data for the inside head area for various cartridges? I can factor out firing pin so the formula to check would be:

max pressure of 43mauser / inside head area of 43 mauser <= max pressure of conversion cartridge / inside area of conversion cartridge

so if <= condition holds then the conversion is ok.

Yes, barrel threads in. Measuring the are of the locking lugs is going to be a pain... (not my gun, so not doing anything with this one ;) )

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A .223 @ 45,000 will produce less backthrust than a .30-06 @45,000, because the area upon which the pressure is applied is smaller.
A 71 Mauser has only a single locking lug. I don't know that I would expect a whole lot from the old single shot bolt action guns like the Mauser, the Chassepot, etc. that were converted into inexpensive shotguns.
I believe that Nick Makinson would make up double express rifles starting with Browning 20ga shotguns. He would add a Greener crossbolt type third fastener. A double rifle may be made up on a 20ga sized frame, but not necessarily a 20ga shotgun action. Obviously some actions were capable of being used for either rifle or shotgun. There is a book detailing making up double rifles on shotgun actions. The author goes into quite some detail about the actions suitable for conversion. Some are, many aren't. There is a member here who has made up some of these double rifles.
 
farshot said:
What were their "not good results"?
Receiver damage? Bbl bursting?
Chiappa makes a kit of sub-adapters to go with their folding survival shotgun, in 20 and 12 ga. They advertise that the adapters may be used in any SAAMI spec break open shotgun.
I find this interesting and would like to know more.
Click to expand...

It's been a few years since I've spoken with them, they called 308 and 223 unsafe pipe bombs and would only use US made 7.62x39 ammo in their adapter for that caliber - they do state that for the adapter they produce on their site. Wouldn't tell me what actually happened to test guns.

The adapters Chiappa makes are all pistol caliber, nothing higher pressure... I've tried them in different shotguns, they are fun. Don't own any, though, thinking of making my own. This whole exercise is about taking the idea further (specifically being able to use swappable extractors on the bolt)
 
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Allow me to stick my neck out here as this (sub cal adapters) is a topic I have Zero experience with.
As with anything on the internet, opinions abound on the topic of rearward bolt thrust. Most discussions center around the cartridge case expanding and “gripping” the chamber to an extent thereby reducing bolt thrust (I recall such a discussion on here about experiments done back in the day incrementally reducing cartridge head support during firing to measure rearward thrust)

Let’s pretend that has truth to it for a second, with a sub caliber adapter the cartridge case will “grip the adapter” but the adapter is free to thrust rearward against the breach face is it not?
 
If that is the case, the adapter would function as a floating chamber, and increase backthrust. Colt .22 Ace/conversion kit, Remington 550 autoloader are examples of designs using floating chambers for that purpose.
The expansion of the cartridge case does tend to reduce backthrust. Don't know that it is something I would want to count on if I were concerned about the strength of the action and its resistance to backthrust.
 
Hmm...I've a '71 in 12 Gauge and a 12 guage to 45 LC adapter.
Best not mix the 2 I suppose.
Maybe after I see the chamber is in spec...brass shells are a thought.
 
Tiriaq, that is my thinking precisely. I couldn't guess at the numbers or if it would have a real world, noticeable effect; but I wonder if it might be a factor.
 
Also by keeping your cases clean and free from oils ( lube) will decrease back thrust , increasing brass hold on the chamber walls....a good thing.
Cheers
Brian
 
tokguy said:
Hmm...I've a '71 in 12 Gauge and a 12 guage to 45 LC adapter.
Best not mix the 2 I suppose.
Maybe after I see the chamber is in spec...brass shells are a thought.
Click to expand...

Is it a takedown (most 71 and 71/84 conversions aren't)? Or is your adapter super short? Otherwise you won't be able to get it in there...

tiriaq said:
If that is the case, the adapter would function as a floating chamber, and increase backthrust. Colt .22 Ace/conversion kit, Remington 550 autoloader are examples of designs using floating chambers for that purpose.
The expansion of the cartridge case does tend to reduce backthrust. Don't know that it is something I would want to count on if I were concerned about the strength of the action and its resistance to backthrust.
Click to expand...

So no real way to calculate the backthrust off the .43mauser numbers? :(
 
Close to 5000 ftlb of bolt thrust.

.475 internal base area (just a guess based off case od and brass thickness)
28,000 psi. ( saami 45-70 trapdoor max pressure)

Most often its calculated with the external base dimension to have some built in safety factor and its way easier. Most lists you find online is using external. Using the OD of the base a 223 has 6800 and a 30-06 has 10,000 ftlb bolt thrust for comparison.


There is case grip. I believe it was P.O ackley that was doing some testing and removed the locking lugs from a rifle and the bolt didnt open when the case was roughed up before firing. Hard to calculate what it would be though. As its based off that case the chamber condition, also the brass makeup and the pressure then you could start calculate the friction.
 
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tiriaq said:
Sure. Pressure and internal base area.
Click to expand...

right, but a cartridge in a regular barrel would not be exerting the same bolt thrust as a catridge in a 'floating chamber' insert, right? So the action may very well be ok with 43mauser as originally chambered but get damaged if it's in an insert. Doesn't look like I'd be able to say "43mauser exerts this much bolt thrust so an insert in a caliber that exerts same or less would be ok" because there's an unknown 'chamber grip' factor that's absent with an insert.
 
The insert would act like a large weight
Think of blow back guns and bolt weight and slowing the accelleration. The chamber insert would do that. The weigh would soak up a lot of the thrust


Or just skip it and use the raw numbers for the case. You can be 100% sure they wont increase when put in the adaptor as your not calculating for the the case grip anyway.
 
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Evanguy said:
The insert would act like a large weight
Think of blow back guns and bolt weight and slowing the accelleration. The chamber insert would do that. The weigh would soak up a lot of the thrust
Click to expand...

yes and no

it would provide more mass and a larger effective 'case head' so it would have a longer impulse over a larger area, but the energy will be the same.
 
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