It's concerning how many times I have spoken to Gunsmiths concerning lengthening chambers and this conversation or information has never came up. Most recently with a fellow who is well known around the PGCA world if you will. I had the chambers lengthened a few years ago on a Sterly 20 and that's it to date. Duly noted and good info Gents !

Think about this ... I am sure that the time between the production of most old SxSs and the birth of collector associations and forums on the internet, bringing these concerns to light, most of the guns we get our hands on today were shot with any shotshells that were available at the time, maybe 100s, 1000s of times without even thinking of chamber length or age of the gun. If someone bought an old Fox in 1982, for example, they would simply go to the gun store in their neighborhood or K-Mart and buy any shotshell they wanted for what they were going to hunt.

Bill

Todd's comment about tapering the junction from the forcing cone to the bore got me thinking. Tapering the junction from the forcing cone to the bore is actually "lengthening the forcing cone". By the way, how much encroachment into the forcing cone is experienced in the first 1/4 inch of the forcing cone? I would rather have the extra metal than 150 psi less pressure.

In the early issues of Double Gun Journal (I'm going on memory here friends) John Brindle wrote some articles titled "Nitro for Black". His conclusion was that if you stay at or under 8,000 psi you are good to go with modern powders.

The VH that Sherman Bell blew up Took some 31,400 psi before it let go at the breech where the extractor pin and the dolls head screw almost connect. If I recall correctly I had those forcing cones lengthened. I know I wanted it done, but I can't recall it the gunsmith talked me out of it or not. Today I will not change chokes, mess with the chambers or forcing cones on a Parker.

Yes. "Lengthening the forcing cones" is what the procedure is called.
The gun is a 6 lb 5 oz 12 gauge, with 30" bbls, that came with 2 1/2" chambers.
Lengthening the forcing cones lowers the pressure, without weakening the chamber area.

So you make the walls of the chamber thinner but it does not make the walls weaker?

Would lengthened chambers void the proof?

Thanks for that question Bruce. Lengthening the forcing cones without weakening the chamber area defies the laws of physics big time.

Here is a Parker published loading table showing drams in black powder. Just substitute drams black to drams equivalent in smokeless. Unless you have hogged out barrels or wrecked stocks.