Proper Forcing Cone Polishing
When altering the forcing cone there is more involved in doing the job right than just taking a reamer, reaming out the metal and calling it good. I take great pains in putting the finest polish possible on the forcing cone after I have lengthened it, a step many overlook or rush through. The fact is that the polishing step is half the job when done correctly. A rough, unpolished forcing cone is a magnet for plastic wadding and residue accumulation, which in turn attracts moisture and causes rust. It also, obviously, makes it harder for the load to pass smoothly through the forcing cone to the bore. With the forcing cone it is important to remember that you get what you pay for.
I have received many barrels for other work that have had the forcing cones lengthened at some point but never polished. Cleaning and maintenance are greatly reduced with the polished forcing cone. I have had customers tell me that they see more pattern consistancy through a long day of shooting after I have polished their forcing cone than they saw before. This must be because the forcing cone is slick and stays clean even after many rounds have been shot through it.
When the forcing cone polishing is done correctly, you should be able to run a tight patch, maybe a piece of an old T-shirt, down your barrel from the chamber end, through the forcing cone and down the bore and never feel any difference in resistance except that the inside diameter gradually gets smaller through the forcing cone area. If you feel a rough, snagging like resistance when the patch passes through the forcing cone then your forcing cone needs to be polished. Try the tight patch test and see how your barrel feels. After I cut and polish your forcing cone it will be long AND smooth. Very Important!
Take a look inside your barrel. Odds are, if it's a regular production shotgun it has a short, abrupt forcing cone. On the other hand, take a look inside an expensive trap gun/ skeet gun, most guns out of the factory custom shops, or the guns that are winning the turkey shoots/splatter shoots/ card shoots, and you will see a nice, long, polished forcing cone. This tells us something!