I have never used a torque wrench on a breech plug, I only tighten up to "snug". If I were to guess, I would say I am using somewhere in the neighborhood of 60 ftlbs.
David
Phew! I was beginning to wonder if I had missed something, and also wonder if the Victorian pistol barrel I am using for my practice piece was safe to use. Finger tight takes it to where the original registration marks are not much more than an eighth apart. It was proofed like that, so it should be fine, but all the same... 
Could I ask, What sort of torque could I expect to encounter if I purchased a barrel with plug fitted?
DaveP,
I hope I will be forgiven for giving modern gun torque limits, but I think it will give us some perspective that many people can identify with – even for black powder guns.
I spent a career using torque wrenches to tighten Gas Plugs on NM M14’s and Gas Cylinder Lock Screws on NM M1 Garands as well as tightening barrels on M1903, M1 Garand, M1 Carbine, M14 and M16’s and AR 15’s and 10’s. So I have a lot of experience what the torque pressure “feels like” from 20 ft lbs to as much as 120 ft lbs (and rarely more) when barrels were torqued on TOO tight. The US AMU used 150 inch pounds ( about 12 ½ foot pounds) to 15 ft lbs to torque on M14 Gas Plugs while we Marines used 23 ft lbs. M16’s/AR 15 barrels are torqued from a minimum of 30 ft lbs to 80 ft lbs, though I have seen a LOT of AR15 barrels over torqued beyond that. M1 Garand and M14 Barrels go from 40 -80 ft lbs. as do M1903’s. WWI British Enfield Barrels were often tightened much more than M1903’s and so much you have to be VERY careful you don’t twist the action out of shape when removing the barrels.
HOWEVER, as a young Sergeant and NM Armorer in the early 70’s, I often witnessed one bolt action rifle shooter who came to most of our matches and he swapped his barrel for a different one when going from the 300 yard line to the 600 yard line. That barrel indexed no more than about 10 degrees from top dead center and he was only using maybe 15 to at most 20 foot pounds of torque when tightening up that barrel. That rifle was in Winchester .308 caliber and he OFTEN won with that rifle so torqued. IOW, you don’t need as much torque to hold on a standard barrel as you do for a gas operated one.
I have pulled maybe four or five dozen original Rifle Musket barrel tangs over the years. When you discount the ones that were rusted in place, I imagine the torque on those tangs were from 50 to 60 foot pounds originally, though some few went up to 80 or so foot pounds. However, we have to remember these rifles fired very heavy .58 caliber Minie Balls and were also probably torqued a little more than necessary to ensure they would not come loose in Military Service.
Before we lost Gary Brumfeld, he mentioned in one post years ago that original barrel tangs on Long Rifles were not torqued near as much as many modern rifles and that includes modern black powder rifles where factories apply more torque than needed for liability concerns. I have only pulled maybe a dozen or slightly more original civilian rifle and pistol barrel tangs, but the torque applied to them was in line with what Gary mentioned. In the rifle tangs I pulled, they probably averaged around 30 – 40 foot lbs at most and pistol barrels went around 25-35 foot lbs. though one or two were less than that.
Heinz is absolutely correct in his statement in the above post that “
The end of the plug needs to seal against the bottom of the breech hole at the same time the shoulder of the tang snugs against the back of the barrel.” This is much more important than a great deal of torque applied to the barrel tang and especially for a single shot muzzle loading pistol barrel.
So on a factory built rifle or pistol, you are probably going to see a greater torque setting than what you may find from some custom builders.
Finally, from the few original British Flint pistols I have worked on and many more that others have worked on, they had a tendency to seem very light as to the torque necessary to tighten the breech plugs. So unless you are getting a gas leak or some other problem, I don’t believe you have a problem as long as the breech plug and tang fit correctly.
Gus
