I know this is a really old thread, but I am especially fond of the 410 and have been reloading the 410 for many years.
I copied this from a post on another forum:
"fiasconva wrote:
Before you give up on that 410 pattern some 2 1/2" shells in it. I've seen several posts about them patterning better than the 3" ones. I have no idea why but I'm sure some of the more knowledgable shooters here can tell you why. Good luck!"
Factory 3 inch shells are loaded with the same short wad/cup as the 2 1/2 inch shells. This leaves a very large amount, percentage wise, of the shot column unprotected to scrub against the barrel, deform, and fly erratically.
The 410 has a very "tall" shot column. The 410 also produces higher chamber pressures. When you have high pressure pushing on the bottom of that tall, therefore heavier per area shot column, you get deformed pellets that fly erratically.
When I learned, several years ago, that the 3 inch shot cup/wad was going to be discontinued, I bought all I could find, several thousand. I have loaded 3 inch Remington SP hulls, lower pressure charges, 3 inch wads, and 11/16, weighed, hard copper plated shot that would give patterns in a 25 inch circle at 25 yards that were very concentric and approaching 75 percent. They did, of course, still have the typical 410 denser centers and sparser fringes, but much, much better than factory loads.
I have loaded 2 1/2 inch hulls at chamber pressures in the 7000-7500 pressure range, 2 1/2 inch wads, 1/2 ounce, hard, copper plated shot that would approach 80-85% at 25 yards.
The copper wash on shot does not make it any harder. The antimony content determines the hardness. The copper wash actually smoothes the outer surface of the shot, making it rounder and therefore more aerodynamic, holds patterns better. The high antimony content helps it resist deformation from pressure and scrub.