I would be happier with fire walking back and forward across the target area rather than getting more accurate onto the specific target tile.
Rather than a burst aimed at one spot I think spraying down the area around the alien is a better use of the burst.
Think Blain in Predator taking down the forest because he saw something move in it, rather than precise controlled bursts from an AR.
That gives a far bigger difference between using single shot and burst fire.
Using the current burst fire round number to limit the maximum number of rounds in a burst does seem to cover all the bases though.
You can use the same actual mechanic for all weapons but some can have their burst limited, to three rounds for the assault rifle for example.
Hopefully then you will be able to alter their accuracy, deviation limits, and strength bonuses to give a unique feel to weapons.
AR bursts can be more tightly controlled than the longer full auto bursts of other weapons to fit them into a single target attack role.
SMG are much less accurate and controllable in full auto but engage closer up.
Pistols are trickier but I would give them a double tap shot in auto that is tightly controlled as they will have pretty poor range.
Machine guns will be powerful up close and dangerous at any range.
They will also have a property damage downside due to the limited control you have over your shots.