Manufacturers go through all sorts of contortions in order to wring tighter patterns out of buckshot. Funky wads, shot cups, plastic buffer material, extra polishing, arranging the pellets in spiral patterns, etc. Most of these are too complex to screw around with without special reloading tools, not to mention patience.
I just got in fifty rounds of the latest Sellier & Bellot 2-3/4" Magnum Buckshot, which features a clear plastic hull. Clearly laid out inside were four rows of three lead 00 pellets each stacked on top of each other. The shell was capped by a clear disk held in place by a crimp. I got out a dental tool and found that the disk was easily removed, permitting the pellets to be shaken out. I did this to a total of five rounds.
I then sliced open seven rounds of Federal "Tactical" buckshot, which gave me a total of 63 copper-plated 00 pellets. It was then a simple matter to place the copper pellets in the S&B shells and replace the retaining disk.
(The point of this exercise is to see what effect changing a single component can have on final group size. Actually, that's not perfectly true, because the Federal pellets are also supposedly double polished, and are noticeably more perfectly round than the S&B pellets off the bat -- at least they roll much more smoothly on a hard surface. But hey, what do you want from me?)
So what does that mean? Yes, going to copper-plated pellets without changing anything else will help.
While I only isolated one particular here, much of what I've been learning over the last several months points to the conclusion that the key to shotgun performance is the degree of pellet integrity, i.e., all other things being equal:
Give me some time, I'm still working on this . . .