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Breech plug length for .32 caliber rifle...

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Sidney Smith

58 Cal.
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The thread below regarding the man's 45 caliber and his short breech plug, got me thinking. When I did the breech plug for my .32 caliber, I had to file off some of the rear of the barrel and cut off some of the plug in order to get it to mate up with the flat I had wanted. I bought the barrel without a plug since I wasn't sure which tang style I wanted. I ended up buying one. It is a 5/8X18 plug. The barrel must've had a plug on it originally because there is a witness mark on one of the flats. This barrel had a 1/2 inch deep thread hole as measured by me. I figure the barrel maker had already fitted a plug on it and removed it when I said I didn't want a plug installed. After getting the one I bought seated to the shoulder of the bore, it measures .420, or about 3/32 less than half an inch. When I took off enough of the plug to get it to the 1/2 inch depth of the hole it wouldn't line up to the flat I had wanted. Instead the former witness mark was 180 degrees away or would have been right on top where it would be seen. Its too deep of a mark to file out, so I filed the barrel and plug until I got the mark to hide under the stock. I'm wondering now if I have enough of the plug to be safe. Trouble is I already inletted my stock and positioned the lock in relation to this plug. I hope I didn't ruin things. Any ideas? Am I ok or do you think this will be an unsafe weapon to fire at this point?
 
I’d feel very safe. I’m shooting some originals. Have a .36 with 3/8” of threads. Very good threads on a new breechplug.
 
Subject to correction from the guys that know a lot more about metallurgy than I do, I generally think the current 1/2" long plugs being marketed for all barrels, are much longer than they NEED to be for calibers less than 50, and that goes even more so for steels tougher than 12L14.

When they do (aluminum) scuba tank inspections (working pressure up to about 3300 psi) they need to have 8 uninterrupted threads in both the valve and the cylinder in order to pass visual inspection. They are usually made with 10, so as to give a little leeway just in case one gets nicked along the way. I think those are a coarse 18 tpi threading as well.
 
If the breechplug face shuts off on the threaded hole ctbr, the only force that will bear on the breechplug is from the .32 cal bore. Area X pressure of the expanding gases equals the force in lbs.. Using .330 dia equals .0855 square inches X 10,000 psi = 855 lbs of force acting on the plug face. That's not a whole lot of force even w/ the stated 10,000 psi. The highest bore pressure is called a pressure spike and is achieved when the expanding gases are overcoming the PRB's inertia {starting to get it moving} and is very short lived. The pressure at the muzzle is a lot less than that at the breech.

Have seen a few pics of exploded, ruptured bbls and all had the breechplug still in the bbl. The above paragraph explains why. Your breechplug should work fine.....Fred
 
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I once asked a machinist friend who was familiar with BP about breech plug pressure and he told me that 3 good threads would hold all the expected pressure, and the rest of the threads were overkill. This was on a 3" bore cannon made from a piece of 3" modern naval gun barrel (chrome- moly) which had threads about 1/2" wide and deep. It had been fabricated before I bought it, and I had the breech x-rayed. The plug was at least 3" deep. The trunnions were welded on, which I've been told is a no-no, but the machinist said heat distortion of metal temper from welding (in this case) only extended about 1/4 inch past the actual weld. (The trunnions were chrome moly too). I'm not a metallurgist, but it seems steel threads can hold a lot of pressure if made correctly. Perhaps some of our members can comment on this, Geo.



'
 
Intuitively, it would seem to me that a coarse 18 tpi threading would provide more sheer strength / resistance than a finer threading like say 24 or 32 over the same distance. (As per the scuba tank inspection thing) there also must be something "magic" about having 8 full threads too. A standard 1/2" x 18 tpi breech plug would have 9. I hope the engineers here know the answer and can chime in.
 

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