The fouling pushes betweenthe gaps between the threads. HOWEVER, this is greatly reduced or eliminated if the liner or breech plug or drum for a drum and nipple seals are either end. Sealing to a inner shoulder is best. Sealing at the far end is better than "open".
Threads that can be seen will have to fouling removed by cleaning.
If I seat anything against a shoulder I seal it by fitting, pointless to use a shoulder otherwise.
Where this is impossible I have taken to using thread sealer.
Jerry mentions the liner flexing to seal the threads. Flex is not a good thing unless the part is made of a material that will not work harden.
This can also be reduced by having a tight thread fit. Modern thread "tolerance" allows fouling in. Making a breech plug screw plate with the same tap used to make the threads in the barrel reduces "tolerance"
I have removed breeches from douglas barrels made from 3/4x16 bolts that were only exposed to the fouling where the vent liner was installed and after considerable use they had fouling back 3-5 threads. This was three different 54 caliber rifles I obtained used. There was no serious rust but fouling did get back in there.
The counter bored liner that I sent to Larry Pletcher tested very well for speed and will produce very low thrust on the liner if properly installed. The test rig, since it was testing different designs made this impractical. I usually make these liners in 10x32. sometimes #12.
In using a #10 or #12 with a .100" diameter hole through the shoulder and a slighter smaller hole for the counterbore there is no chance of a fouling or oil trap if the liner is FITTED tight to the shoulder.
Calculate the area of the .100" hole (3.141 times radius squared) you get .007 square inches. 12000 psi times .007 is 84 pounds if my math it correct. This means that even on a fairly light barrel a shoulder can still be used. It does not take a thick of shoulder to seal the threads or more than 4-5 threads to hold the pressure.
If we use the full tap drill diameter for 1/4 x 28 .213 we get 132 pounds. Plus the 1/4 removes more metal from the breech. So for about .054" increase in diameter we get a about a 50% increase in pressure. Yes the 1/4" is stronger but it also requires a larger hole in the side of the barrel and is more likely to expose breechplug threads.
The square inch thing is why it is so difficult to blow out a threaded part. Look at the tensile on even a low grade 3/4" bolt and compare to the thrust generated by a 54-58 caliber rifle.
I have tested a "soft" 3/4" x 16 plug with 4 threads engaged in a 4140 54 caliber barrel using 200 gr of FFFG swiss a patched RB and a 54 maxi over that with no leakage or movement. This is a pretty heavy proof. I don't recommend 4 threads, but it held.
What does all this mean??
It means I like to seat pressure bearing parts to a shoulder.
Why?
The breech plugs mentioned above and because a rifle I built about 1968 was leaking gas (call it gas cutting) around the "removable" vent liner threads by 1977. This was a 58 with a Numrich barrel that was supposed to be 1 1/8 but measured 1 3/16" so barrel wall was not an issue. Liner likely came from DGW.
So I tend to seal the threads on threaded parts using a shoulder. In most cases it will make a stronger part.
Back about 1967-68 I had a drum break off and penetrate a window when the rifle I had restocked was fired. Yes I failed to support it with the lock plate, I was a kid without anyone to tell me any better, but I have only made one D&N gun since and that about 28 years ago (it was well supported too) and won't make them at all now. Funny a lot of vintage guns have unsupported drums and they apparently did not break off. But I ain't gonna say why.
I am kinda a perfectionist. So I tend to do things the hard way. I get worse as I get older. A lot of what I do concerning barrels and attaching parts to them has it roots in some experience of mine or that of a friend.
Yes there are LOTS of old guns out there with small diameter breechplugs, 4-5 thrreads, not sealed and they never failed.
But some did, at least one was written up by a physician since he removed a breechplug from the man's eye socket after he suffered from horrid headaches for a year of more (this was in a magazine back in the 1980s but he had it in his head for several years IIRC) after his rifle burst.
I am SURE the man that breeched the barrel had done a lot of them the same way and never had one fail. There was no real detail about the exact nature of the failure so the barrel may have burst or bulged and expelled the plug as a result.
Dan
Breech plug being installed in a 50 caliber GM smooth bore barrel. Note it seals the bore. Yeah it takes longer.
