I hit on this same question myself, but when the Mutants & Masterminds site posted their preview of the 2nd edition combat rules, I was in heaven.
URL is:
http://www.mutantsandmasterminds.com/super-vision/000213.php
In summary, a weapon with Autofire allows the hero to employ four different options: drilling a lot of shots into a single target, spreading shots across several targets, laying covering fire to protect a single ally, or laying suppression fire to inconvenience a single target. The rules for M&M are quite straightforward - for instance, driling a lot of shots into a single target means that whatever amount by which you beat the target's Defense becomes an increase on your damage bonus against the target (thus, increasing the target's Toughness save DC). With a caveat that if the target is immune to a single shot for whatever reason, it's immune to a mass of 'em. Simple and brilliant.
I've considered adding in a fifth option, from d20Modern: use autofire to affect all targets within a 10x10 area, as an area effect (Reflex save to avoid damage; my variation would probably have the DC equal 10 + combat bonus or ranged attack bonus).
As for feats, I wasn't anticipating requiring any feats to use Autofire, but that's based mostly on setting flavor. It might make sense that a character needs training to use autofire weapons competently - might divide Weapon Training into (martial) and (firearms), or make firearms martial weapons and make (autofire) an additional category - again, depending on the setting.
I've got no particular insight into bursts - i'm not planning on using burst rules unless as special features of particular weapons. For example, a gun that allows the use of burst fire for +2 to the damage (or more), but doubling what I call the "whammy range" for jams, misfires, and running out of ammo. ("Whammies" in my games are bad things that happen when you role a natural 1, or higher if there's an extended whammy range. This is what I use instead of tracking ammunition, and I often encourage my players to describe their own whammies during play. Obviously, I enjoy a very fast-and-loose approach to my games.)
The issue I see debated regarding the interaction of armor and extra hits - damage bonus vs multiple Toughness saves - is, I think, mathematically a non-issue (at least to a limited degree). If we were to assume that a target was hit by a long burst of, say, 20 bullets, and the armor provides protection against each and every one of these bullets, then either A. the GM rules the attack wholly ineffective because no shot has any hope of penetrating the armor (
very rare) or B. the defender effectively rolls 20 Toughness saves - which statistically gives the same result as rolling a single save against a damage bonus that has been increased by +20.
In all probability, at least one of those saves would be the worst possible result, one would be the best possible, and most of the ups and downs woudl come out about even. Since we only care about the results that are under a certain figure, we simplify. (One difference: rolling twenty saves could conceivably accumulate a lot of Hurt results, skewing the results further toward losing Health levels and dying right away; rolling against a massive damage bonus makes autofire slightly less deadly - a character may go straight to dying, but not to dead.)
Make of that what you will, I guess.
No kitten is going to tell me what to do. I'll fall off this branch to my death if I feel like it!