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The .450NE 3.25" at 44,000 psi chamber pressure shows approx 10,000 pounds force against the breech.

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The .450NE 3.25" at 44,000 psi chamber pressure shows approx 10,000 pounds force against the breech.

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What formula did you use to calculate the back thrust? My calculation is a bit less, namely 7542 lbs. of force. My calculation is thus: Area of inside of case (in^2) x Peak Pressure (lbs/in^2). Specifically, a .450 Nitro case has an inner diameter of appx. 0.462 in., with an area of .1676 in^2. operating at 45,000 psi. Also, this does not take into account any adhesion of the cartrdge case to the chamber walls: this will tend to diminish the total back thrust by an unknown amount.

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Hi Carpetsahib,
That's a good question, and one I'm happy to answer.
I use the standard proceedure to calculate breech thrust, that being area of base in sq inches, times pressure in psi, but I never use the case ID, but prefer to use cartridge base outside diameter.
The reason for this is that in the event of a case head separation, the breech thrust needs to be calculated at base outside diameter, not inside.
Case separations do occur with reloads in DR's from time to time. This is not related to the cartridges being loaded to an excessive pressure, but rather that the cases have been reloaded one time too many.

Regarding the case gripping the chamber walls, as you say, the amount of grip, and the amount of diminished breech thrust is an unknown amount. That is one of the reasons I don't factor it in, because there is no way of knowing what to factor in.
Another reason I don't include it is, again, in the event of a case head separation, there will be basically no grip and therefore no diminished breech thrust.

When calculating breech thrust, with safety in mind, I prefer to do it as a worst case scenario.
This means (for me) using base outside diameter for the calculations.

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4seventy,
What I am trying to do is understand the forces that are generated during firing, and the stresses that the mechanism must withstand (else we will all be "hoist with our own petard"). To that end, I have calculated the forward thrust on a barrel as a result of a projectile upsetting in the rifling, and friction as it travels in its mad rush down the tube. The formula for that is Tf=Cpsi*Ab*Cf, where Tf is Total Forward Thrust; Cpsi is Peak Pressure; Cf is Coefficient of Friction of the bullet against the barrel. The Cf that I use for my calculations is .36. This is much greater than the generally accepted figure for Cf of Cu vs. steel. My Cf tries to account for the engraving and upsetting forces as well as straight friction.

Here is an example of my calculation: .303 bullet, Area = .0706 in^2, Cf = .36, Cpsi = 65000, Tf = 1778 lbs.

Another example: .375 bullet, Area = .1104 in^2, Cf = .36, Cpsi = 65000, Tf = 2584 lbs.

These numbers seem reasonable to me, at least for design purposes. Please comment on my approach and let me know if you think I'm on the right track.

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I received this email with some photos of a BSS DR conversion. I think these are from member "Canada."

Thanks for sending these.
The email included the following details.........

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I can't seem to post photos so I will try to send them to you. I hope it works and perhaps you can post them. You can see the bluing wear where I carry the gun. The barrels are winchester model 64 and were fitted to a mono block. It was done over 20 years ago by a gunsmith who has passed on

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Edited by CptCurl (04/02/11 09:59 PM)

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Carpetsahib,
Not sure of your formula of the goings on in the barrel, but your thoughts on chamber pressure are spot on. If its 44,000 psi inside the 450 nitro, then its that pressure per square inch of inner surface area.

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Case head separations are to be avoided at all cost. The 'time' when the case separates is the deciding factor in whether the action blows up or not. This pertains to bolt actions as well, not just weaker double gun actions.

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What I'am saying is that separations can occur in DR's due to TOTALLY different reasons to what happens when "Harry the Handloader" overcooks the load for his bolt action super-boomer, or overworks the brass with a badly adjusted resizing die.
To fully understand the difference, you need to have done a LOT of double rifle shooting and a LOT of double rifle reloading, particularly with bottleneck DR cartridges.

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Case head separations caused by very high pressures stretching the case, are VERY different to case head separations in low pressure cartridges fired in double rifles.
Double rifles are a totally different ball game in that respect.
What I'am saying is that separations can occur in DR's due to TOTALLY different reasons to what happens when "Harry the Handloader" overcooks the load for his bolt action super-boomer, or overworks the brass with a badly adjusted resizing die.
To fully understand the difference, you need to have done a LOT of double rifle shooting and a LOT of double rifle reloading, particularly with bottleneck DR cartridges.

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I would really like to add a greener cross bolt but it looks pretty tight for room on the action. If anyone new where a picture of one of these converted Brownings was I'd love to see it.

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There is a Mackinson BSS .470NE double on the Lewis Drake site at present.
It has a third fastener added, but not the Greener crossbolt system.
There are some nice quality photographs there, and well worth a look.

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