Its a 12 bore. I was thinking that lengthening the chamber / forcing cone would allow me to shoot 2 3/4 shells without cracking the stock. I have decided to leave it alone and order the 2.5" RST ammo.

Thanks for letting us know what gauge it was.
The chambers are not 2-1/2”. They are 2-5/8” which parker i tended to shoot 2-3/4” shells from.
So, shoot 2-3/4” shells. Shooting 2-1/2” is only to be cautious, which is fine too if you want.

Sherman Bell, DGJ:

Here’s what I use. The eights are quail or target shot and I use sixes for pheasant, last week limiting on wild not released pheasant.

Or a person can always use the loads that Parker intended to be used:

I too like the Remington STS light target loads for trap, but prefer them in 1 1/8 oz. (1145 fps).

Is Equal enough?
 

Drew:

I get your point that “The cone can be lengthened (BY AN EXPERT, who is removing metal) while maintaining a wall thickness equal to the end of the chamber”. And those are very helpful graphics you provide to illustrate the result.

But would not GREATER, not equal, wall thickness tend to be called for when the reducing the area that a powerful charge has to travel in a tube? There is, after all, greater FORCE that is created and has to be contained by a “Forcing Cone”.

Even if lengthening the chamber is going to actually reduce pressures, as Bruce says, maybe by 300 psi, are these trade-offs feasibly calculable so as to determine net benefits when dabbling in invasive surgery on the most important part of any gun (the barrels)?

I know that this is a recurring topic and may well be tiresome for some. But since it relates to safety, i.e., the avoidance of maiming oneself and others, and can have consequences that risk besmirching the reputation of the entire sport, it is worth a solid understanding.

Russ: a couple of very helpful graphs.

This from 1931 shows the pressure-distance curves with vintage powders; FFg, Ballistite (Dense) and Schultze & DuPont (Bulk) and DuPont Oval "progressive burning smokeless powder"

http://pic20.picturetrail.com:80/VOL.../409804775.jpg

The pressure is falling at the forcing cone, though less so with DuPont Oval

Modern powders peak and fall much more rapidly so the pressure at the forcing cone is even lower

http://pic20.picturetrail.com:80/VOL.../412289224.jpg

So there would be no safety justification for more wall thickness in the cone compared to the end of the chamber (where pressure is higher)

And to clarify Bruce's point, Bell's study compared 2 1/2” chamber with a 7/16” forcing cone vs. 2 3/4” chamber with a 1” forcing cone and showed the pressure decrease was about 400-1200 psi depending on the load and vv. the pressure rise could be as much as 1200 psi
A summary is here about 1/4 way down
https://docs.google.com/document/d/1...vwLYc-kGA/edit

For the forum members that are not aware there exist 2 types of reamers. There are chamber reamers that lengthen the chamber and reduce the taper of the forcing cone and forcing cone reamers which lengthen the forcing cone only. Your eyes, hands, and gun proceed with caution if at all.

Y’all are way to smart for me. I just run light 2 3/4 shells through my Parker or I stock up on RST shells at a shoot. If I’m shooting a non-Parker 16 ga with 2 1/2 chambers, I buy 2 1/2 shells. It’s pretty simple, but I’m a simple guy:) Lengthening the chambers costs money, decreases the value and don’t do a dern thing for the shootin. Ain’t gonna do that.