DB asked me to respond, so I'll give ya a shadetree view of this.
The chamber is the cylindrical area of the rear of the barrel. It does not include the tapered portion in front of this cylinder, known as the forcing cone.
The C model was typically chambered for early shotshells which generally spec'd at slightly less than 2 1/2" when OPEN. Since the shells were made with an overshot card and a roll-crimp, their outward profile looked similar and was similar in length to a modern 2 3/4 in cartridge before firing. However, modern shells have a star crimp which, when opened upon firing, extends the length of the casing to just under 2 3/4".
That extra 1/4" or so will open in the area of the forcing cone rather than the flat, cylindrical area of the chamber. This does a couple of things. First, it constricts the shot coming out of the shell when fired raising chamber pressure and also increases shot deformation...the shot at the rear of the cartridge is crushed by the pressure from the explosion of the charge and it's restriction in the forcing cone. In the worst circumstance, you have a dangerous chamber pressure. In the best, poor patterning from deformed shot.
The most effective fix is to lengthen the chamber and extend the forcing cone with a reamer. A Clymer is probably the best although they are a bit pricey. If you don't want to do it yourself...and most folks don't, then a competent shotgun smith can do the work for $30 to $35. A wise investment.
The work will reduce felt recoil, save you the labor of cutting and roll-crimping shells and improve the shot pattern from your 97.
Good luck!
