Yes, "ordinance" refers to how the gun is fired, "xyz template" refers to the effect at the painful end of the thing
Ordinance 1 / Large blast < typical shot, say from a basilisk, scatters
Heavy 1 / Large blast < same template effect, but shot is a direct hit, with no scatter
There are a few very small (well, the models are quite large actually) exceptions to this...
Ordinance 1 < direct shot at a target, use the ballistic skill of the shooter but when penetrated, can be annihilated (vanquisher guns, heavy rail guns), some of these appear twin-linked too, cannot move and shoot typically, and cannot fire other weapons unless special rules allow