vent liners and thread engagement

[J.D.]

The "barrel tester" thread got me to wondering about how wall thickness and the number of threads, in a vent liner, relates to a safe installation of said liners.

Bookamundo mentioned that several liners he tested blew out, so what is everyone's recommendation for the minimum number of threads, on a liner, to engage  the female threads of a barrel?

Lets say a white lightning liner with 32TPI, or a 1/4-28, or even 3/16-24, for example?


Thanks, and God bless,
J.D.

[Dphariss]

Large diameter vent liners are subject to a lot of pressure.
They also weaken the breech.
Stainless steels are not as strong as many would assume when used in shock loading applications. Barrels made of "free machining" stainless (416 in this case) have been failing in modern guns and at least one maker does not recommend them for temperatures much under freezing...
I have a number of 303/304 alloy liners in use. I have never had a problem with the ones I make but over the years have had one of the "removable" types (14x28 IIRC) gas cut past the threads, this could have been builder inexperience this rifle being made in 1968 or 69. I had a store bought crack internally, this was found in pulling the cleanout from the patent breech it was in. It is possible I over torqued it but its just as likely it failed due to material and construction issues. I replaced it with a similar shop made with thicker walls but the same material.
I will avoid barrel steel comments....
Many things can increase the possibility of blowing out a liner.
If this is a concern make one that is 10 or 12x32 thread and install as shown in the "cupped breech test" thread in the shooting forum. If seated against a shoulder in the manner shown pressure to the liner is FAR less than if the vent is just installed by drilling into the bore with a tap drill and then taping the hole. Calculate the area in square inches of a .150" to .30" diameter circle and then multiply by the approximate PSI then do the same for a .100-.125" circle.
I do not like diameters over #12 in barrels due to the amount of metal removed from the breech. In flint patent breeches this is less a concern depending on how they are constructed.
With small diameter liners 10 or 12 x 32 tpi 5 threads engaged will be strong enough if properly installed against a shoulder. 6 or 7 is better of course and can be done on most heavier barrels. Since the liner seated against shoulder with .102-.105 hole only has this much pressure expose it is FAR less likely to blow out that a 5/16 diameter vent that is
If the barrel wall is too thin to allow at least 5 threads in a 32 thread then the barrel needs a patent breech... These can be made to look like the barrel easily and makes for a much stronger breech for thin barrels.

Dan
Just for giggles....

.105" circle will bear 135 pounds at 16000 psi.
.250" (about 5/16 tap drill) circle will bear 785 pounds at the same pressure.
Breech thrust can be found in a similar manner.
This does not tell the entire story but its representative.
You math wizards out there may feel free to critique this in case I have "erred".

[J.D.]

Thanks Dan, that is a big help.
God bless,
J.D.