What do they do, say when i send my gun to briley and have the forcing cones lengthened?
The older guns designed for cardboard/felt wad types had forcing cones that went from a chamber diameter on a 12 ga, of around .800 to a bore diameter of .725 pretty rapidly. My best guess is that transition took place in about .030-.050".
In the 80's, a lot of shooters had the forcing cones lengthened so that that transition took place over a longer length. Early on that was a length of about 1 inch, to 1.5 inches. I understand that some are even longer now.
The process is pretty simple. Get a reamer for lengthening forcing cones. Put the barrels in a vise. Attach the reamer to a large T handle, put some cutting oil in the forcing cone area and on the reamer. Run the reamer in the chamber end and start cutting by hand. The reamer will pretty much stop cutting when the old forcing cone is gone. I have an old Clymer reamer I bought in 88, collecting dust.
At that time, there was a whole process of barrel work that took place that was suppose to help reduce recoil and improve patterns.
Lengthen the forcing cones
Back bore the barrels---Std bore diameter then was .725-.730. The entire length of the barrel was opened up to .740-.745. That opening process was stopped just prior to the choke area. Then the chokes had to be opened to give the choke the proper constriction, or put in screw in chokes.
Barrel porting also came into vogue
Starting in the 90's, Beretta, Perazzi and others started producing competition guns that came from the factory with all those goodies plus adjustable comb stocks. Most gun makers also started making 3" chambers standard about that time.