Shotgun got one, so it'd be unfair to leave our rapid-firing friend from the party, even if it's already kinda powerful. Straight to my suggestion:
Dunno how many of you are aware of this, but the original's chaingun has this nice thing that it always fires at least two shots, and that those two will always, always hit the mark, or whatever you're aiming at. Thus, I'm suggesting the following:
a) Make the first two shots of the series of five to always hit, regardless of you being on Coward or anything else. The enemies will also benefit of this bonus.
b) If after the firing there will be no movement, picking things up, or any other activity from the player, including getting hit by the enemy, then all the subsequent shootings will be done much faster (like you had Finesse skill on two or something), but also have larger spread and less chance to hit - and the two first shots won't always hit this time, either. So like, if I fired the enemy, and then immediately fired again (and the foe won't hit me between the two times), I'll be able to do so muchos faster but with less chance to hit. Same goes with all the later shootings, until I get hit or decide to do something else.
c) Also give the weapon an alternate firing ability that will always fire exactly two shots - and those two will, of course, always hit. This'd be convenient against weak enemies such as zombies, against whom the five bullets is kinda overkill. This "sniping" won't benefit of the faster firing bonus mentioned above. Each point in Triggerhappy will add one bullet on this.
So what do we have here? Either you can do "short, controlled bursts", that won't really do much damage but they will always hit, or you can go all-out like "DIE YOU FILTHY DEMONS DIE DIE DIE DIE", and bombard the enemy with a massive barrage of hot steel. Neither of these would make the chaingun much stronger of what it is now, but give it some new dimensions, extra coolness, and classic Doom feeling.
Your thoughts?