THis discussion went on earlier (don't remember who exactly was stating it, but it made sense). If you have the identical burn rate, and the same case dimensions, same weight bullet but different caliber, the larger cal will always go faster out of the muzzle. Take a WSM case that is getting around 63,000 psi. The 270 WSM has far fewer square inces to have force excerted on it than the 300 WSM
. Therefore the 270 will not get the net force that the 300 will if using the same weight bullet.
However, you spoke of short range balistics...
the smaller diameter bullet of two bullets of the same weight and same design and different calibers will have a higher balistic coefeicient causeing it to carry better and eventually out preform the shorter fater bullet that is going a little faster out of the muzzle.