Author Topic: Chamber and Breech Pressures (Read 714 times)

Bigl · « **on:** November 17, 2004, 09:15:34 AM »

Okay,
for this question lets consider straight walled rifle cartridges such as 444 Marlin and 45-70 Govt..

Would I be wrong in thinking that the greater % of pressures would be consumed in the chamber walls, leaving a very minimal % of pressure at the breech face?

Bigl

Iowegan · « **Reply #1 on:** November 17, 2004, 11:22:37 AM »

No, from physics theory, the pressure on all chamber surfaces including the breach face should be equal. The thickness of the brass really has nothing to do with the exerted pressure.

As for case wear from pressure, the weakest point is where the case wall meets the web of the head. The case mouth usually shows splits before any other failures. This is because the mouth is crimped which causes metal fatigue. How the brass case behaves may mislead you in thinking the walls have more pressure.

Judson · « **Reply #2 on:** November 17, 2004, 01:01:01 PM »

Pressure per sq inch is for all intents and purposes equal on the inside of the case. However since on a non lubricated case, the case adheres to the chamber walls the breach pressures may be substantially lower. P.O. Ackley did tests proving this fact by removing the locking block from a 94 winchester and after some minor modifications firing the rifle with no locking system at all. Unless the case was lubricated the action did not open.
This is interesting but not anything to try when holding on to a gun or when within schrapnel range. For all practical purposes the pressure is (per sq inch) the same in all directions other then against the base of the bullet.
One more thing, Ackley was not using a cartridge of very high pressure, if he had the head of the case would have blown off and some very interesting would have happened as did latter in the testing when they used very high pressure loads.

Bigl · « **Reply #3 on:** November 17, 2004, 02:03:35 PM »

Hey Fellows,
this is a follow-up from an earlier post I made about rifle barrel inserts for shotguns.

The reason I'm asking questions about chamber and breech pressures is because MCA Sports out of Alaska builds rifle barrel inserts for single shot and double barrel shotguns. Most of the calibers they have available for these inserts are pistol cartradges, but some of them are rifle cartridges. Such as: 30-40 Krag, 30-30 Win., 375 Win., 45-70 Govt., 444 Marlin, 22 Hornet, 7.62X39.
Now some of these cartridges develope some pretty good pressures, yet MCA Sports states that their inserts for those cartridges are perfectly safe to use in a shotgun thats in good shape and has good tight action (locks up good). Infact, they stated that one of their test guns is an older standard H&R 12ga. single shot shotgun. Cartridges they test fire with is standard factory loaded ammo.

So at this point I'm kind of puzzeled. MCA Sports, which has been in business for several years building these inserts say it's safe, and has had no problems with their product.
On the other hand, replies I get back on the forums from gunsmiths and others say they are skeptical concerning such devices in a shotgun (won't handle the pressures).

So, who's right and who's wrong or who really knows??? WOW, what a can of worms!!!

Bigl

Iowegan · « **Reply #4 on:** November 17, 2004, 02:47:10 PM »

This is an entirely different thing. The insert becomes the chamber and the shotgun chamber is nothing more than a container for the insert and an easy way to use the existing trigger / hammer / firing pin. All the pressure will be contained by the insert. A typical shotgun chamber is made to handle 20,000 lbs of pressure even though the loads don't run that high. The breach of a single barrel shotgun is way stronger than the chamber itself. A 12 gauge shotgun chamber measures about .8". A 45-70 chamber measures about .48". This gives an insert's wall thickness of .16", plenty thick to contain the pressure of a straight wall case. The other cartridges you mentioned have even smaller diameter cases so they would have thicker insert walls. Add the strength of the insert to the strength of the shotgun chamber and you would have a very strong system.

Bigl · « **Reply #5 on:** November 17, 2004, 04:51:30 PM »

Iowegan,
I can see your point concerning the chamber strength, but what about the pressures of the cartridge face against the breech face of the receiver? I'm kind of questionable about the breech face handling the pressure. Unless the pressure at the breech face is considerably lower than the chamber area. If this would be true, than I don't think I'd have a problem with using some of those inserts. I like the idea of turning a single shot or double barrel shotgun into a multi-use-gun.

Thanks,
Bigl

Iowegan · « **Reply #6 on:** November 17, 2004, 06:50:16 PM »

Not to worry, the breach face is much stronger than the chamber and will take considerable pressure. I wouldn't try this with one of those cheap Brazillian made guns but it should be quite safe in a quality shotgun. Also, I wouldn't shoot hot loads. Stay within the factory ammo specs and you won't have a problem.

gunnut69 · « **Reply #7 on:** November 17, 2004, 07:37:21 PM »

I am very skeptical about the high pressure rounds.. the large diameter firing pins on the typical shotgun would crater badly and in some older guns where the pins are a little worn the could well be a pierced primer. Also I have a very nice little Ithica 12 bore in the shop. It was being used with standard factory ammo and the frame cracked. It seperated where the water table and the standing breach meet. the crack continues nearly all the way around the bottom of the action.. If this would face happened with a rifle round in the chamber the actions opening would have been quite hazardous indeed.. In a modern gun built of steel it may be a great idea but in any of the older weapons made of caast iron, your looking for trouble.. and the large diameter firing pins will also create problems.

BoarHunter · « **Reply #8 on:** November 19, 2004, 12:04:42 AM »

In Europe, many cheaper double rifle guns are made on 20 gauge shotgun action and they stand up OK providing the action is a good one (SKB for ex.).

Rounds used are low pressure cartrige like the 8X57JRS, 444Marlin.

So a 12 gauge would be even stronger and the insert will take up the radial pressure.

But as Gunnut said, the issue may be the firing pin that if too long or dented could pierce the primer leading to gas spliting the stock and may be injuring the shooter.

Good double rifle have double escape holes around the firing pin.

The other issue is convergence of the rifled barrel (or two) and no sights !

There is now plenty good quality combination guns or double rifle that makes buying these inserts not worthwhile.

Judson · « **Reply #9 on:** November 19, 2004, 12:35:29 PM »

You can build double rifles on SOME shot gun actions but not many. For one thing you need some sort of third fastener, such as a Greener cross bolt. The firing pins must be bushed if they are large in diameter and those two holes around the firing pin on some shotguns is for a spanner to remove the bushing and firing pin not gas control. For the most part double rifles have rather poor gas control and a blown primer can wreck both the gun and you. NEVER, repeat NEVER use a shotgun frame for building a rifle chambered for high pressure cartridges!!!! As for American made shotguns for the most part, forget them they do not have the strength to handle double rifle work. Even such guns as Parkers do not compare to such as Scott or Greener. One more thing pressure is pressure and even a small high intensity cartridge such as .223 can blow your gun. You have to look at the pressure per square inch of the guns original proof test compared to the pressure per square inch of the intended cartridge. Figure out the square inches of the head of your intended cartridge and figure out the pressure per square inch then compare this to the pressure created by the original chambering. This will tell you if you are within the pressure limits of the gun.