Military rifle barrel length

[Jack]

Spike, the small rimfire cartridges (small in powder capacity) often give maximum velocity in around a 16 inch barrel. Reason for that is, there just isn't much powder to burn, so the powder is all burned in a relatively short barrel.
That is usually NOT true in centerfire cartridges with a larger powder capacity. In most of the typical centerfire cartridges, like the 270, 30-06, 30-30, barrels longer than 20 inches will almost always produce higher velocity than barrels under 20 inches.