Lengthening the forcing cone reduces the deformation of the shot while it is getting from the chamber to the narrow part of the barrel. The longer you make the entryway the gentler it is to the shot. Two inches works fine and five inches works too. No blow back that I am aware of. HMB.
I've personally looked at maybe a dozen shotgun barrels that have failed to the point there was a large hole in the side (my gunsmith collects them) and after looking at the barrels I agree with him that the culprit is a poorly done job of lengthening the forcing cones ---- either not done "straight" or to much metal removed.
I think any effect of "doing" the cones on any change in patterning is probably less than the normal shot to shot variation in a regular barrel and there are maybe 3 or 4 gunsmiths I'd trust to do the job and none of them are cheap.
Actually.. the Kolar.. and the new Perazzi 18.7 bores leave little room for improvment..as both have learned from our barrelsmiths what really works.. Some of the older guns could have improvments with better designed forcing cones.. designed for todays plastic wads...
This 870 barrel was pulled out of stack of similar ones because they have a reputation for short and rough forcing cones. I had several which looked just like it. In other words, this was in no way a "selected specimen" or anything similar. It's probably just like the one on your 870 in the safe, the one you put away when you got something a lot better.
When I ran 10 Federal Papers (lot 681) through it is was clear that it was a match for - and better than, actually - any barrel on any gun I owned.
I had the forcing cone lengthened by someone who seemed to me, as I watched him, an expert in the task. With a piloted reamer he did the job then spent a lot of time polishing it and making it look just great.
But it made no difference.
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This is why it frosts me to read as I did above, that "it will make a difference." Have any of the people who say that ever tested it? I've done it three times now and there was no difference in all three cases.
"Your mileage may vary" but it would surprise the heck out of me if it did.
This mid-1970's Ithaca Perazzi - called RP for the person who campaigns it - shoots just great with its old, non-trendy bore, forcing cone, and 0.040 choke. In spite of these weaknesses it won a handicap at the Grand a few years ago.
Compared to a lengthened-forcing cone Perazzi of a similar year and similar dimensions, it does just fine. There's no evidence that the modification did the other gun any good at all.
No.. I think it's a combination of things.. forcing cones alone will not change a whole bunch.. backboring alone.. will help reduce hot spots in the pattern.. as the bore gets larger.. part of that relates to how the large bore opens the forcing cone.. and how the choke section is handled in the backboring operation..
Many of my older Perazzi barrels.. have a HOT core.. Most of my larger bore barrels.. later or worked on..have a more uniform pattern.. The percentages may be similar.. but the uniformity seem to be improved.. Now.. as I shoot only 24 gram.. I'll stay with my old.. hot core barrels.. That's what works for me.. If I shot 1 oz.. or 1 1/8oz.. the new 18.7 bore would be my cup of tea.. Now.. I do have 1 barrel 18.7mm that's factory virgin.. After I fit it to the receiver Rich Bullard sold me.. I'll report back how it patterns..if anyone wants to know.. It's .024/.032