Here's an old attempt at a regular ballistic burst fire assault rifle, by yours truly.
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I thought I remembered this not sucking so much....
It's not very reliable, you're probably better off using (a mod using) some Lua method.
But it does have truly interrupted bursts, with separately fired bullets, and can be aimed between the bullets of a burst, something no other .ini-only mod can do.
There are 3 easy ways to half-ass it, but you won't get quite the right result.
1) Null tracers. This works like L4D2's SCAR, where it's really still 'fully automatic', as you can hold down the trigger for multiple bursts, but it has brief delays between bursts. So it isn't really burst fire. The main problem with this method is that if you stop firing after a burst but before the delay tracer, when you start firing next time, you'll start with the delay, which is obnoxious.
2) 3-Bullet rounds with separation. This makes all three rounds appear at the exact same time, but they appear in front of the muzzle at a random distance. A problem with this is that bullets are appearing too far away, and this means you can shoot through walls and 'miss' at point-blank. Also, you can't change your aim between firing the rounds of the burst, because they all appeared at once.
3) Emitters. If you fire an emitter that then fires three bullets, you get an effect of firing three bullets, but because this emitter is not attached in any way to the actor, you can't change your aim in between shots, and it doesn't move with you quite right. In extreme cases, you can start firing, and then walk into your own line of fire.