mountPoint - where the player holds the objects or weapon. This would be places in the middle of the handle, or where the gun would be held. If you were using MS3D, the handle should be a 2x2 in the small grid.
muzzlPoint - where the gun is fired. This would be placed right where the bullet comes out.
(optional) ejectPoint - (I think that's what it's called) is where the shell, or bullet casing debris leave the weapon.