Just tried out 2.3 (fairly recent build). Looks like the shot count for at least MG is changed. Anyway, you are right, I should focus on 2.3.
Also, editing the new sounds for various shot counts is minimal work. Literally, it takes maybe 5 minutes to extract the sound file, load it in Audacity, edit it (=select the area to be repeated, repeat it X times with the Repeat command, add the original single shot as the final round), save, and add it to the pk3 file. And there aren't that many weapons in UFO:AI or that many different shot counts. In fact, can anyone list the weapons with burst fire capability? Assault Rifle, Machine Gun, and Machine Pistol come to mind at first.
This is what they did in Jagged Alliance 2 (v1.13, which is heavily revamped JA2). There are separate sound files for each shot count and for each weapon. Some use, of course, same sounds. Burst bullet count goes up to 50 there, so that means 50 sound files for a gun. And might I say, there is a s***load of guns in that game now.
"If the interval between shots changes"? Doesn't seem very likely that it would ever change. Why would anyone change the interval between shots for a weapon?
I'm telling you, this is totally one way to overcome the clumsy sounding short burst sounds. It would absolutely guarantee steady sounding bursts and it's less work than you think. Sure, it might take extra few minutes of future modders' time but that's all.