Tapering? Lengthening? What's the difference? Both are removing metal.

Do we even know what gauge the subject gun is???

There's no "Proof" in the US.

There maybe no proof houses in the U.S. but manufacturers would be under constant litigation if they chose not to prove there guns.

I was referencing the H&H in the post. It came to me with 2 1/2" chambers. Briley's recommended tapering, as opposed to lengthening the forcing cones.
They told me that tapering the cones would not weaken the chambers.

Yes, how does one taper further what is already tapered without removing the material involved?

My crude rendering here of what I understand of Parker gun barrels (as per THE PARKER STORY and Larry Del Grego and Son).

Because the angle of the cone exceeds the angle of the external taper of the barrel, there is more wall thickness within the cone than the end of the chamber.
The cone can be lengthened (BY AN EXPERT, who is removing metal) while maintaining a wall thickness equal to the end of the chamber

Courtesy of Hollowell

http://pic20.picturetrail.com:80/VOL.../414147079.jpg

Lengthened chamber with the blacked in portions being the metal removed in the process.
Upper side shows a 5 degree angle to the cone. Lower side shows the cone cut parallel with
the outer wall yielding a cone length of approximately 1 1/2".
Courtesy of Miller Fulks as posted on DoubleGunBBS

http://pic20.picturetrail.com:80/VOL.../414167872.jpg

This is a cross-section of a chamber lengthened from 2 5/8" to slightly longer than 2 3/4".

http://pic20.picturetrail.com:80/VOL.../414172909.jpg

This MAY NOT APPLY to small gauge or light weight British 2 1/2" chamber game guns.

Bottom line IMHO is a 12g No. 2 frame "shooter" Trojan with a 2 5/8" chamber could safely have the cone lengthened by an expert.
I'm not aware of any benefit in doing so.
I would NOT mess with the chamber or cones of a short chamber 20g.

As I think you can see, among those most knowledgeable, if your gun is deemed safe to shoot by a competent gunsmith, have fun and shoot it. No need to mess with the chambers.

The photo of provided by Drew depicts a badly chattering reamer and the taper does not appear to be concentric with the bore.

Early in my Parker education I bought a Trojan 16 with Lengthened Chambers. It’s probably worth half what I paid, and the the guy that lengthened is out his cost to alter. Aggregate loss of value 500-750 even 1000 dollars.

The gun would have handled correct ammunition fine. RST or Reloads using published Hodgdon 2 1/2 inch data.

William