I patterned my 20ga Winchester 1300 with 20-pellet #3 buck earlier this week. #3 buck contains .25 caliber pelelts. At 75ft, the pattern just about fills a doorway from side to side and looks pretty sparse. At 20ft the pattern is about the size of a human head and the wad ends up somewhere around the edge of it.
The point is that where I live 20ft is the longest shot I am likely to take and 8ft is far more likely, at which point that spread would be under 3 inches. Moreover, in order for the target to get all of the load, I have to center it on the desired point of impact, so aiming is helpful.
The plus side is that even a 20ga with buck or slug is more formidable than a .357mag or .45acp and less loud. The down side is that slugs may well travel through the perps and the wall behind and the next one. Birdshot that is so often recommended appeared not to penetrate sufficiently in my very informal tests.
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Oleg "cornered rat" Volk (JPFO,NRA)
http://dd-b.net/RKBA