In Mutants and Masterminds 3rd edition, on page 25 of the Hero's Handbook, the rule for balancing attack bonus and effect rank is as follows;
Attack & Effect: The total of your hero's attack bonus and effect rank with that attack cannot exceed twice the series power level...
For many situations, this is a simple rule to apply. How accurate a hero is with their power limits how potent that power can be. As it is an innate part of their hero, they are simply limited to this restriction by the rules of the game, and the in-universe justification for this is that that is simply how their power happens to work.
How does this work when their 'power' is a hand gun, and they decide to just pick up a bigger hand gun?
While the rules discuss equipment points being spent on 'standard gear,' there seems to be no explanation on what happens when a character acquires a weapon they Should be proficient in using, but has a higher damage output than the weapon they normally use. This becomes quite important in the case of hand guns, of which the game has multiple that deal different damage numbers. All of them being handguns would mean they should all fall under the same skill for knowing how to aim a handgun. The issue being that larger hand guns can be bought (there just is a perk that states the character is varying degrees of wealthy), acquired from allies who happen to prefer a larger gun, or even obtained from downed enemies.
Would the new weapon reduce their effective accuracy because of its higher damage? Would the damage that weapon does be reduced because of how accurate they are? Would they just be weirdly unable to use the weapon? Should the amount of hero points a player can spend on accuracy with handguns be limited based on the highest damage a handgun could have, regardless of whether or not their character actually uses that hand gun? I am uncertain, and could not find a definitive answer.