Hey Tellerve,
Sounds like a cool idea, first off. Yeah, firing the gun while running to hide behind things is a staple of action movies.
In order for it to be truly effective, sure, you'd want to have Shot on the Run, preferably with Strafe and an autofire weapon as well, so that your single shot could strafe along a 20' section of cover to keep the bad guys pinned. But failing that, here are a couple of options:
1) A single feat that lets you do mostly what you want. Call it something like "Wild Shooting" and have it require firing at least five bullets and target four adjacent 5' squares. One random square receives an attack roll (which means that this works best against a LOT of opponents, so you KNOW you'll have a chance of hitting somebody). The attack carries a -2 penalty to hit, and the opponent is treated as having full concealment (since our hero is firing blindly). This means that, really, there's a CHANCE he'll hit something, but it's not a great chance.
2) No feats, just flavor text. That's usually what I'd go for, unless I were running a gun-centric campaign. Here's how I'd work it:
Our hero wants to move across an open space, starting with cover and ending with cover. Let's assume that he's moving only a standard move to get from one place to the other.
Now, really, the player is doing some good roleplaying here, because unless opponents readied actions to fire upon our hero, he's fine while making that run, due to d20 Modern's turn-based system. The willingness to lose some ammo just to provide a bit of protection against something that isn't likely to come up anyway is good roleplaying in my book, so I'd reward it. I'd have him be able to fire, say five bullets (or an autofire, if his weapon has one) while running, but instead of treating those bullets as attacks, I'd treat them as one Intimidate check against all opponents in a 90-degree cone in one direction, adding the hero's base attack bonus as a circumstance bonus to the roll. On a success, opponents cannot target the hero while he runs to his next cover, even if they had prepared actions to do so. On a failure, opponents can continue to target the hero, but there is a 20% chance that any opponent who targets the hero could be struck by a bullet (attack with a base attack bonus of +0 and apply no additional damage from weapon specialization or other skill-based factors -- the only attack or damage bonuses that apply are equipment bonuses from having armor-piercing or otherwise improved bullets). This intimidate check automatically fails against opponents with damage reduction that would render the bullets useless or an utter disregard for their own lives (like a robot or golem programmed to keep attacking without checking for its own safety).
I'd even let our hero use this if he was running from one area to another, and couldn't get there in one move. On round 1, he moves and does this ammo-based intimidate check, and if he succeeds, nobody in the four squares he "targetted" can attack him on their next turn, either by firing or by running up to him. They can attack other people normally (unless those people are within ten feet of our hero, running along with him), but will probably stay behind cover. On round 2, then, untouched by enemy fire, our hero does a double-move and makes it to cover safely.
So, cinematically, he's pinning people down by firing in their general direction, and in game terms, he's either keeping opponents from attacking or, if he's really really really lucky, doing a bit of damage. Is he in any danger of outshining somebody with Shot on the Run? Nah. Is it going to change the outcome of many fights? Doubtful (20% chance of making a +0 attack on an opponent who has probably got cover in addition to his normal defense, which likely means that he needed a 20 to hit, so 20% x 5% = 1% chance of actually hitting anyone). But does it sound cool? Yep. And that's what it's all about.
Dunno. I'm sure that other folks have options as well, and really, as long as the players are happy, that's what's important.
