Like all games, Hero System fails to match the reality of firearm combat in number of important areas. Unlike most games, 5th edition admits that it this on purpose, even if while doing so it's overstating what little attention to reality it did use. The most obvious example is the fact that handgun damage will only kill (or even incapacitate) with a single shot in very rare cases. In HERO, the typical handgun would be lucky indeed to drop a straight up average man on the street who didn't have even a single point to spend on his stats.
A general discussion of firearms in rpg design can be found here.
While HERO System is too grainy to handle all the complexities of firearm combat. It can do a much better job than it does straight from the book. By changing a few things not only would suspension of disbelief would be easier to maintain, but the system would more accurately reflect many of the genres it was intended to in the first place.
Firearm Damage in the HERO System has three basic components. The actual killing damage (say 2d6-1 K for the .45 ACP as given in the rulebook), the stun modifier (+1 for the .45 ACP in the book) and the effect of certain advantages and limitations
Unless I was willing to resort to major rule additions and changes, I would be limited to modifying these factors to produce the results I wanted. Fortunately, most of what I needed was in place. The exception was the far too grainy effects of the armor penetration limits which I needed to expand.