Thanks for the replies.Paul S, I was thinking pretty much the same as you along the lines of case capacity and pressure.Then there is the short barrel factor. I would think the amount of powder you could burn in a barrel would depend on the length of the barrel.Say you were to fire a30-06 and a .308, both loaded to the same pressure with the same bullet,out of a 12" barrel.Would the 30-06 be faster because of greater case capacity?
The short answer is yes - the greater the case capacity the faster the bullet - with everything else being the same. All the powder is burnt in the first inch to three inches from the chamber. Only the heat and pressure remains to push the bullet past that point. The larger the case capacity the slower the powder is so the longer it is at FULL pressure. Slower powders (if there is enough room in the case always give LONGER pressure curves and produce higher velocity.
If the 3006 and 308 were loaded with the same amount of the same powder, they would be very closely matched for velocity but the 3006 would have a lower pressure because of the larger case - with lower pressure the powder would burn a bit slower but it would produce a longer pressure curve. That is how it would get the same velocity with the same powder charge of the same powder with less pressure.
At first glance it would seem that the round with the highest pressure would get the highest velocity but it is the length of time it gets the pressure that is most important not the pressure.
The proof of that is right in your loading manual - the faster burning powders are sometimes loaded to a higher pressure but they produce lower muzzle velocities.
Anyway - the 308 and 3006 loaded to the same pressure with powders best suited to the case volume and firing the same bullets will always show higher velocities in the 3006 (not much but it will always be higher because there will always be more powder in the 3006 and a longer burn.
