D20 has rules for Burst and Autofire, at least Burst fire requires a feat to use.  I'm not sure if autofire does.

Many are critical because of for likely obvious 'balance' reasons, burst fire actually is less likely to hit, and does more damage.  As I understand it, this bothers individuals actually trained to preform the maneuver, as the primary purpose is to actually increase the chance of getting at least one hit out of the burst, potentially extras for a more devastating effect.

My suggestion is, to instead have burst fire use up a pre-determined amount of ammo[lets say 5], if they hit or not.  The first roll does not represent the first shot, it represents the most likely shot.  It would actually get a bonus to hit of 1/2 the number of extra shot expended.  [5-1=4;4/2=2]  So an individual gets a +2 on their first shot.  Burst shots are subject to a higher chance of botch. 1-2 on the first shot.  (I'm thinking about increasing the chance of botch for each iterative shot)  IF the first shot hits, you get to re-roll a second attack roll against the same target.  This roll takes a penalty equal to twice the iterative shot number [-4 for the second shot, -6 for the third, -8 for the fourth]  You only get to make subsequent shots if the prior roll succeeds.  In an example, the first shot in a burst gets a +2, the second is rolled only if the first hits, gets a -4 instead of +2, the third is only rolled if the second hits, and has a -6 to hit.

Also, I am inclined to actually decrease the damage caused by a single hit of burst fire, due to its being less targeted.  Also, burst fire prohibits application of abilities that cause precision damage.  [you can't burst fire to get sneak attack damage]  However, any hit achieved in a burst could count at a critical, and the burst damage reduction would not be applied to the critical damage.

I imagine that botches could result in the gun jamming, or could result in non-enemies or valuables to potentially become targets of a stray shot.  If there are individuals adjacent to the target, or the line of sight path between the shooter and the target, they could potentially become unintended targets of a burst fire..

My understanding is that burst does allow greater chance to hit but an extended burst actually kicks the weapon beyond alignment.  Like a shot gun it is somewhat an area denial weapon.  While it is line of sight the damage could be spead out hitting any in that line starting with the first/closest and working back. But each would have a maximum of maybe three dice or hits, leaving the remainder for background targets.
So if you shot a three burst and all hit (I don't know what is normal but would base it on how much over your to hit was).  Then nothing goes beyond. But if only 1 hit then to exist to hit but I would give the second to hit a penalty, also making multiple hits less likely.
If you did a burst seven that would leave more to hit the tail. If you burst at a military unit unless you roll a critical failure I would spread one each across the group.