Breech and chamber pressure: safety of Chamber Inserts

[cpileri]

BLUF: Is it safe to use an insert that 'converts' a larger-bore-lower-pressure cartridge to a smaller-bore-high(er)-pressure cartridge?

Details:
I was looking over the available Shotgun Inserts, esp here: http://www.mcace.com/shotguninserts.htm as i have bought from Mr. Dube in the past and been pleased (*). They are solid steel and fit well in the chanber/barrel of the host platform. i like them and the idea of them ALOT and want to expand my use of them...SAFELY!
Now I am wondering if a couple specific conversions are safe in guns of good working order:
- 410bore-to-218 Bee or 22 Hornet: the 410shotgun pressure is comparitively low at ~13000 psi. Whereas the 218 Bee and 22 Hornet are 40-45000 psi.
and,
- 20ga-to-30-30WIn:
with the 20ga at ~12000psi and the 30-30 at 32000psi.
or, similarly,
- 12-ga-to-45-70Govt at ~11500 and 28000psi, respectively.(**)

The larger caliber above conversions have a lesser difference (absolute and percentage-wise) in pressures between the 2 chamberings, but also have more massive projectiles and powder charges.

Given the surface area differences in the breechplate of the break-open shotgun design versus the new, smaller surface area cartridge rim, and the smaller cartridge higher chamber pressure but now contained in a sleeve of solid steel (the insert); what are my safety concerns?
Anyone have real expertise in the area?
anyone use one of these inserts without problem?
with problems?
thanks!
C-

(*) out of ignorance perhaps, i have always used conversions that stay in the 'low-pressure' range: example: a 12ga-to-22LR. Even though 22LR is approx DOUBLE the pressure of a 12ga; iit is so small that the hunk of steel insert looked ample to contain it. I have also tried 12-ga to 38SPL and used the target wadcutter loads; but have not yet dared to try 357 Magnum in the insert.
(**) for that matter and if it helps, the MCACE insert for 12ga can also be made in 444Marlin which has pressure at 44000psi and similar dimensions to the 45-70, so it may give some indication to the strength of the steel.

 

[Nicodemus38]

what the insert is is a COMPLETE rifle barrel with chamber that is machined to fit into the internal contours of your shotgun barrel. the inserts are in fact an actual rifle barrel, and if you had money and time and proper equipment, you could turn them into perfectly fine single shot rifles on your own.

as designed, the insert absorbs all chamber pressure from the cartridge.

 

[cpileri]

ok

Thanks, that makes sense intuitively.

Now, how about the pressure from the cartridge rim pushing back against the breechplate/face?
What i mean is: a 12ga is designed to handle a relatively large 12ga rim pushing backwards when fired and spreading that momentum over the surface area equal to that of the 12ga rim. But the 444 Marlin pushes back not only harder (or does it?!?!) but into a smaller surface area.

Is that safe? or dangerous to the breechface over time? the locking mechanism?

As for the 'or does it?' comment; is it pressure or momentum that determines the stress the action, or in this case, the breechplate experiennces?
Pressure is obviously higher in the 444Marlin.
But momentum, if we assume its equal to the momentum of the slug moving foward, compared for each is:
12ga: 1 ounce at 1500fps= 0.02835 kg x 457.2 m/s= 12.96 N
vs
444 marlin: 240g at 2350fps= 0.01555kg x 716.3m/s= 11.14 N

Which maybe isn't so mush of a difference, though it is still concentrated in the smaller 444marlin rim surface area.

Can anyone explain the physics of what actually happens in the firearm to me?

C-

 

[Virginian]

I didn't check every combination, but a cursory check of some numbers says that in no case does the loading on the breech face or lockup mechanism match or exceed that exerted by the original shotgun chambering. The momentum isn't the biggie, it's the pressure. As far as the ability of the breech face to resist the punch/shear effect of the cartridge base, since that force is not enough to flatten the lettering in the brass I guess we will have to assume the steel in the breech face is thick enough. When taking into account the strength of carbon steels it doesn't really take that much.
To get the shear stress, you take the pressure force over the area. The (pressure times the area of the cartridge base), divided by (the circumference of the base of the cartridge times the thickness of the breech face). Of course this assumes the cartridge is rigid, which it won't be by the time the pressure gets great enough to start to shear thru the breech face, but since it doesn't get to that point it works for analysis, and ignore the fact that the case grips the chamber under pressure. Say you have a 1/8" thich breech face and a 1/2" diameter cartridge base, and 40,000 psi pressure.
(40,000 x 0.19635 sq inches) / (1/2" x 3.14158 x .125") = 40,000 psi shear, which really isn't a very big number for good steels.
Calculating the forces on the lock up and hinge pin is too complicated to do for free , but the same principles apply. And then the locking mechanism is going to see shear and bending.... it gets complicated.

 

[cpileri]

loading

Sir, what did you mean by 'loading'? Pressure? momentum? or some other measure of stress on the breech?

Thanks!
C-

 

[cpileri]

ah

Thanks for the edit, that makes more sense.
SO, bottom line: you wouldn't worry?

You'd shoot a 444Marlin or 7.62x39 in a 12ga insert?

C-

 

[rcmodel]

I think it's safe to assume they are safe or the company would have been sued out of existence a long time ago.

If they caused gun or shooter damage there would have been megabuck product liability lawsuits already.

This is America we are talking about here.

rc

 

[Virginian]

What's that round someone cooked up; basically a magnumized 45-70? Ask them if it's safe for that or a full house Beartooth 45-70, and if they say yes let 'er rip.

 

[justashooter in pa]

figure the chamber pressure multiplied by the base area, and compare. with 7.62X39 you are looking at 50Ksi X (3.1416 X .22 squared or pi X r squared) for 7602 pounds pressure, against a 12 gauge with 15Ksi X (3.1416 X .35 squared) for 5772 pounds pressure.

hmm, there must be something more to this question...maybe you can't get 30 AK inserts.

 

[cpileri]

30 ak

Sure you can, under the 12ga list : 7.63x39.
Does specify American made ammo only though.
C-

 

[Nicodemus38]

however dont you have to take into consideration the recoild force exerted upon the hinge pine?

 

[Shrinkmd]

Uh, silly question here, but the website doesn't really say, so:

Can you use these Mcace inserts in an 870 pump, or only in a double or under over type of shotgun?

The 38 or 45 acp sounds interesting. Any pictures available of how the setup works?

 

[justashooter in pa]

the inserts are designed for single and double guns. some makers produce inserts that are quite long (chambermate). if you get an insert that is 3" long, you could single load it in a pump gun, but i wouldn't try to feed a series of them thru the action, as the cartridge might pop out of the insert.

the 30AK insert specifying "american made" prolly has a 308 bore, as opposed to the .311 that is common in foreign made guns.

 

[Nicodemus38]

american made ammo may refer to the fact that american companies like to use .308 diameter bullets in european calibers with '7.62" in them, though europe uses the british method of bore, and needs a .311/.312 bullet.
ironically if you use metric deisgnation, the enfield rifle in ".303 british" is a "3 line infantry rifle like the mosin nagant in 7.62 (.312 dia bore)

also, american companies seem to load european cartridges to a lower chamber pressure then europe does. something to do with some peoplefeeling european companies failed to do proper proofing to european standards, or feeling euorpean standards are to generous for the metals used.