Originally Posted by
Syme
You probably know that various rifle calibers tend to attain an maximum velocity out of certain barrel lengths, with anything longer or shorter causing a lower muzzle velocity; the same is true of shotguns. Most 12-ga. loads attain full velocity within 12 to 14 inches of barrel, which is why even 10"-barrel breaching guns can get full or near-full performance out of standard ammunition. Anything beyond 14" or so is just lost velocity due to friction with the bore. It's not that the velocities drop in 20" barrels specifically; velocity starts dropping off well before 20", but for obvious reasons, you rarely see shotguns with barrels under 18" being tested in velocity comparisons. If you did, you'd see that just as 22" barrels get less velocity than 18" barrels, 18" barrels get less than 16" barrels, and 16" get less than 14".
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