The muzzle velocity of a firearm can, all else equal, be increased by lengthening the barrel of the weapon which can theoretically be extended up to the point that friction and air resistance would overcome the benefit of pressure buildup in the barrel.
I think that the range of a howitzer be increased simply by adding more explosive propellant. Why is this not used? I would think that this would be a straightforward way to increase the gun's effective range. Any ideas?