My results shooting soft lead unplated OO into a caliche rock wall at 5-25 yards (pellets recovered about 3-4 inches in the soft rock wall) have me agreeing with Bake. Most if not all pellets didn't deform all that much. OO in FBI ballistic gel penetrates up to 24 inches, which is twice the amount of penetration the FBI says is adequate for defensive on people. So if copper plated buck penetrates deeper, I don't want it. And as far as keeping tight groups go, there are plenty of non plated buck shot rounds that are made with wads designed to constrict the pattern.
Remington managed recoil OO buck patterned tighter and had more pellets in the center of the pattern than various other standard 2 3/4 OO out of my 20 inch Mossberg 500 persuader. Of the standard OO 2 3/4 buck I shot, Federals (F 127) grouped the tightest.
Hornady makes a OO buck with their "versa tite" wad (same thing as federals flight control wad) with an advertised velocity of 1600 fps marketed as their OO TAP FPD line. I would assume at that velocity with a wad constricting group there would be deep penetration. It also obviously grouped tighter than any standard OO buck I tested and better than Remington's reduced recoil OO.
Hornady also has a 2 3/4 #4 buck with the same versa tite wad at 1350 fps marketed as a "varmint express" round. It grouped tighter than standard OO, but not as tight as the hornady OO with versatite wad. At 25 yards all of the pellets landed on a torso sized target Generally smaller buck shot spreads faster than larger buck shot all other variables equal.
12 Ga Mossberg 500 persuader. 20 inch barrel cylinder bore.
Sellier&Bellot OO 9 yds- 5.4 inch spread.
Royal OO 9yds- 4.75 inch spread
Remington OO 9 yds- 4 inch spread
Military OO 9 yds- 3.75 inch spread
Federal OO 9 yds- 3.6 inch spread
Remington reduced recoil OO 9 yds- 4.15 inch spread with 1 flyer. Without flyer- 2.4 inch spread.
Hevi shot heavy duty #4 bk 9 yds- 14 inch spread.
Remington "ultimate HD" BB sized tungsten bronze (not lead- more dense= more penetration) 9 yds- 10.25 inch spread.
At 15 yards federal OO was still under 5 inches.
At 15 yards remington managed recoil OO was at 7.2 inches and again without 1 flyer would have had a 4.5 inch spread. Other OO rounds started opening up to 7-10 inches at 15 yards.
I don't have the numbers for the hornady OO and #4 buck with versatite wad but I remember they both grouped tighter than the other buck shot rounds and the OO was the tightest of all (and from other research I've seen, the hornadys versatite wad still does not group quite as tight as federals flight control wad).
Given all that, I live in an apartment in the city. The hornady varmint express #4 buck has an advertised velocity higher than any other commercial #4 buck I've seen at 1350 fps (which means more penetration than other #4 buck, but still not as deep as OO), has the versa tite wad (which means tighter groups) and is cheap and available at Academy on a normal basis.. So that is my choice for HD. I have the Hornady tap FPD OO in the side saddle.
I didn't test any #1 buck bc there was none readily available. Hope that helps bc it was a pain to type on my phone.