Its not really a mod if the weapon gets its own range char; that is a weapon type. As Bull defined, there is no provision from what calibre of weapon you are using. Also, as it was pointed out, an MMG and a Battle rifle IRL use the same calibre or round.
The problem is with how the rules stand, it is now cheaper and easier to make an MMG out of a Battle Rifle, and said weapon now gets a +1 power. Instead of working off the MMG as the basis for the battle rifle, we instead have something pulled out of the air which doesn't fit the existing weapons structure.