Barrel Length vs Power

[mrerick]

in effect, the longer the powder burn gasses expand into the barrel (which is effectively a piston sealed by the bullet until it leaves the barrel) the more the mass of the bullet will be accelerated.

Rich, the relative gain per inch decreases. Between 4 and 5 inches it increases velocity 55fps... Between 7 and 8 inches, only 16fps. This is because powder burned fast, and is not burning as the bullet reaches the longer barrel ends. A slower burning powder would accelerate things differently. The total generated gas pressure is also filling a longer and longer expanding volume until the bullet exits the barrel.

There are many things that impact this. The type (burning rate) and amount of powder, the bullet's gliding surface (including the material and it's bearing surface on the rifling) and the length and bore condition of the barrel itself.

All things being equal, the longer the barrel, the longer the gasses will push the bullet's base and accelerate it's mass. That will result in higher velocity. If the entire powder load has not burned by the time the bullet leaves the barrel, some of it will burn outside the barrel and not contribute to accelerating the bullet.

When you use a chronometer to measure bullet speed, you usually see quite a variation between individual cartridges built with the same components and powder load.

Bullet mass times velocity at impact becomes the amount of terminal ballistic energy that is delivered to the target.

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Thinking all this further, is it any wonder that Daimler invented the internal combustion engine. It's like a barrel with a bullet that never leaves the party...