I use two types of shotshells: CCI shot capsules for my 44 Spec./Mag loaded with a charge of Unique and handloaded shotshell loads in 45 Colt (I use a charge of Unique followed by a lead gas check-flat surface against powder charge-then enough shot to just about fill the brass case, and finally topping that off with another gas check-concave surface toward lead shot-followed by a roll crimp).
Years ago I read where #12 shot supposedly patterned better and killed snakes more reliably than the #9 shot usually used in these shells. After quite a bit of informal testing on potatoes, watermelons, etc., I found that the #12 did indeed pattern better. It would follow that it is also a better killer.
Finally, in answer to Jeffly's two questions: No, neither the shot nor the plastic capsules hurt the barrel or rifling in any way. Second, I have found 15-18 feet to be the optimal effective range for these loads.
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The real democratic American idea is, not that everyman shall be on a level with every other, but that every one shall have liberty, without hindrance, to be what God made him . . . H.W. Beecher