In 1981, I was attending an instructors school run and staffed by the FBI. The weeklong course was supposed to upgrade proficiency amongst the instructors and those taught by same. This being Md, most of the slots were filled by politicians rather than line officers, but a few of us managed to squeeze in.
FYI, the brass didn't trust me a darn bit. They needed some decent shooters, and I'd gotten some attention at the range and in the training office.
So I made it, instead of somebody's Brother In Law.
My oldest 870 was there also,halfway through the transformation from GP field gun to "Serious" use. The bbl had been trimmed,peep installed,cone done, but no extension. During the shotgun phase of the training, the head instructor and I talked about slug shooting. I was doing better than most there, and was delighted to have a few days getting paid for shooting up Govt ammo.
The instructor told me that they had discovered an extension generally tightened up slug groups nicely IF the extension was clamped tightly to the bbl. I shot one of his shotguns in a fast and short COF with the 1 oz Winchesters we had been supplied with, got a nice group and noted the ease of recovery.
So, I went to Guns R Us and got a 2 shot extension for the 18" bbl. Next stop the range, and those Winchesters started falling into groups not much more than half the size of those before. I tried the 1 1/4 oz Rottweil Brennekes that were my slug of choice there and then and watched them go into less than 3" at 50 yards, and oft much less.
Same thing for the couple of other 870s I've done or influenced them being done. Less kick, greater accuracy, every time.
A caveat or two...
One,snug the collar up with the padded channelocks/one click method. The idea's to take out all the play in the unit. THEN use the clamp, and do not oversnug there either.
Two, the most accurate slug before may not be the most accurate after.The bbl harmonics have changed greatly. You may have to retest to find the best combination.
HTH...