Why do firearms of the same barrel length and caliber have varying muzzle velocites?

Question

I have a project to do, and i need to understand why different guns, of the same barrel length, using the same type of ammunition, could have varying muzzle velocities.

If you would like to know, im comparing the M1903 Springfield and the M1 Garand which both have 24 inch barrels (as far as i know), and both fire .30-06 ammunition. To me, the only obvoius difference between the two firearms is that the M1903 is bolt action and the M1 is semi-automatic.

Any helpful answers would be appreciated.

Answer 1

You already hit the nail on the head, if considering identical ammo. The semi auto action uses energy from the round to load the next round. The energy used results in a lower muzzle velocity. The bolt action contains all the energy and focuses it on the projectile resulting in higher muzzle velocities. The extra energy is made up by the shooter cycling the action to load the next round.
Beyond the firearms differences, factory ammunition can vary a good bit from one round to the next. If you look at ballistics charts, many will have a minimum, maximum, and average velocity. This is one main reason for people to reload their own ammunition. They can make it much more consistent than the factories do.

Answer 2

One would expect the M1 to have a slightly lower velocity, as some of the gas from the cartridge is used to operate the reloading mechanism. The friction of the bullet in the barrel will vary depending on the bore size and shape, which will cause some variation in velocity. Except for these variations, the same size cartridge with the same size bullet and same size powder charge should give similar results in different weapons.

Answer 3

What you're studying is called 'interior ballistics'. Some of the things that affect muzzle velocity are (in no pareticular order of importance):
bullet weight.
depth of lands and grooves and their rate of twist.
quantity and type of powder charge used.
And, as you mentioned, type of action.

HTH

Doug