Will a longer barrel give a better pattern?

Your question has been answered. +1 on what trap&field says.

74bronc said:

By better I mean a tighter pattern at a longer distance. I am kind of disappointed by the patterns I get with my 18" barrel at 25yards and was thinking that a longer barrel would give me some better patterns.

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Nope. Should not make any noticeable difference. What trap&field says, however, is correct. A longer barrel will likely help smooth out your swing when shooting targets etc. It is an old myth that "long barrel = tighter pattern." Part of this was promoted by the fact that goose hunters used to prefer guns with extra long barrels, e.g., 36" or more. (Heck, maybe some still do). But it was not because they had any tighter patterns, but rather because the longer barrel gave them a longer sighting plane and, more importantly, a smoother swing when pass shooting geese.