Simple answer - Generally speaking, the shortest length you can use for your application. Why? We are strong believers in maneuverable weapons. The shorter the barrel, the less weight and momentum on your firearm. Unfortunately, the non-simple answer is quite complex, as it depends highly on a number of factors including but not limited to:Legal Requirements:The Federal Government has set the minimum legal length of a barrel at 16" before it is considered a "short barrel rifle" or SBR.