I often encourage customers to pattern their shotgun before they bring it to me and then again after I lengthen and polish the forcing cone. The results have been very positive, with improvements in pattern configuration and gains in pattern density of up to 12% or more. Simply put, you get more holes in a 30 inch circle at 40 yards with a long polished forcing cone than you do with a short, abrupt forcing cone. For the last 20 years my customers have agreed!
Also, until recently I thought that lengthening the forcing cone gave its best results on smaller sized lead shot. I never promoted it as something that could also affect the pattern produced with buckshot. More and more I am realizing through feedback and test results that I have seen that lengthening and polishing the forcing cone also has a dramatic affect on buckshot patterns. I have seen the evidence and heard the feedback from many sources. One of my customers who is producing custom/exotic buckshot loads for 20 ga. shotguns is using barrels that I have done the forcing cones on. I have seen the results that the long forcing cone produced for him and it is interesting reading. I only mention it because most people don't think of the forcing cone as something that you would do for buckshot, or I didn't, but the proof is there. You can read about it at www.custombuckshot.com If you go there, be sure and come back here when you are done! He has linked to my website and I think it is educational.
Over the last 20 years I have been surprised at some of the forcing cone work I have seen. Some are chewed up by chatter and many/most that have been lengthened are barely polished or not polished At All! The polishing process is half the job. In addition to complete bore and chamber polishing, I decided to offer the forcing cone polishing service for forcing cones that have already been lengthened but were never properly polished or were never polished at all. This started out as a "try it and see" type of thing. I knew that from a maintenance standpoint, the forcing cone needs to be polished, but I didn't know for sure if merely polishing a long, but rough forcing cone would enhance performance measureably. The feedback was surprising at first, but after many years of doing this now, the positive feedback has become common. Check your forcing cone. If it is already long but it is rough, and if you are looking to tweak your barrel for every hole you can squeeze into the pattern, this is the final frontier. You may be surprised at the results.