Sorry for some not-so-stellar expressions, but itīs a bit too late for me to make somehow clear what I mean, while trying to be straight and simple to understand AND be a kind of Cicero...
From what I have yet put together to now:
The torque is determined mainly by the ratio of the bore to big thread area, area of the shoulder in front of plug threads which is in contact with the breechplug face, barrel and breechplug steel yield strenght and breechplug thread pitch.
(Yield strenght-pressure/tension, under which a plastic deformation becomes permanent. In most steels it corelates with tension under which the plastic deformation starts to occur-the most important point for fruther paragraphs)
The object of a barrel maker should be as not to let any gases into the thread upon firing, since it may leave there some residue, attract moist, start corrosion and after corroding out some of the material of the threads, compromise the seal-ability (and later strenght) of the breechjob.
The sealing is done by the fact that the small diameter of the breechplug thread is a bit bigger than the groove dia of the barrel-this is made by proper selection of the thread to caliber and by the shoulder on the front end of thread, where the breech face contacts barrel.
Now-upon firing, the pressure buildup presses back against the breechplug with considerable force, using primarily the area given by crossection area of the bore profile. With the plug just screwed into place (like by hand or small key), thereīs enought force to cause a plastic deformation of the plug-which backs at the face a little, leaving a gap into the threads and letting the gases to push against whole frontal area of the plug-not just the bore area. This can be serious and deadly problem, since the area (and forces at given pressure) can be much higher. Exactly to ratio of bore area:big dia. of the thread area.
The backing of the plug consists of "shortage" of the plug (caused by the pressure) and some bend/yaw of the thread ridges upon load. Small moves, but enought to let the gasses (and pressure) into the thread.
So the main object is, to build such a tension in the thread, which "compresses" the plug face against the shoulder with a bit higher force than which the possible charge can produce (on given bore area). So the plastic deformation is permanently there, to make it greater needs more force than the given load pressure can produce on the bore area.
This means that upon firing, the breechplug face does not move backwards any tiny bit, remains firmly seated against the shoulder and no way it let any gasses behind the breech face and into the thread.
The torque is then determined by thread dia. and mainly pitch-with smaller pitch, smaller torque needed, coarser pitch-greater torque needed. As far as I know, torques for defined pull-load in bolts (or push-loads in breechplug) of given thread are tabelated for construction uses.
Then itīs simple math.
Whatīs $#*! of no-easy is, to fit the breechplug in such a way that the real force is applied at the breech face-to-shoulder contact area and NOT at the breechplug to back of the barrel joint (the one vissible on the outer surface). This joint should be just as tight as to be just closed, there should be no tension between the plug and the back of the barrel to speak of, just enought not to let the rain in.
Rattly plugs in old barrels can be from that the gasses could flow into the thread and/or the thread had incorrect preload, so upon each round fired, the thread was bent a bit back and then returned. This led to fatigue and you can bet that the thread geometry is a bit different now (pushed a bit back) then when it was new.
The tolerance and fit of the male/female threads can help a bit to solve this-the closer, the better
(Well, I have heard about an original which had the poor breeching and threading job solved by a hemp string. The owner happily shot it several times a month for several years, since it was otherways in pristine condition, until decision for a very throughout cleaning and removing the breechplug. Rest of the story said something about a kind of major case of upside-down stomach...)
It seems to me that pinning the barrels with such thin pins as one can see on originals comes from calculation that if the plug threads give up, the wood in the wrist area and the breech itself would hold in place enought so the pins get sheared and the barrel would fly forward. The chances to survive a gas blowout, blasted a bit to sides by the breechplug are way higher than recieving the breechplug and wrist slivers directly into eye. As well, this idea seems to me consistent with the fact that swamped barreled rifles are those of higher end of the spectrum, thus probably breeched by more skilled and caring machinist.
I hope this wonīt bring some confusement, I think this would need some pictures to show some what-ifs, because writen it can be too abstract for someone without some basics of Material Science. Also, I may add a disclaimer that above paragraphs are not how-to; it does not mention the relationships between the preload (by breechplug tightening)+loads upon firing, itīs effects on tension and forces in the thread and breech portion of the barrel and consequences for barrel wall thickness etc. If you donīt see it, take this post as a partial, incomplete information.
EDIT
I should have wrote "Strenght and Flexibility" instead of Material Science...
