How does burst or auto fire work in real life?
1) It fires multiple shots.
2) It makes it harder to hit after the first shot because of the barrel jumping.
So if the first shot was easy then each subsequent shot would be a little less easy.
If the first shot was difficult then each subsequent shot would be a little more difficult.
3) It’s also good for suppressing / pinning.
4) They jam.
So how to model that in a game?
You figure out the roll you need for the first shot to hit and then add a penalty for each subsequent shot. It could be simple like a DM-2 in all cases or vary by range e.g. DM-1 at point-blank range, DM-2 at short range, DM-3 at medium, DM-4 at long, DM-5 at very long etc.
So a character fires a burst of 3 from their automatic rifle at short range needing 6+ to hit with the first shot, 8+ with the second and 10+ with the third. Or only roll as long as they hit i.e. if the first shot hits then roll for the second and if that hits then roll for the third.
At long range against a target needing 10+ to hit then the second and subsequent shots would be 13+ which is an automatic miss so you don’t roll for that (unless you’re using the same roll for pinning).
A submachinegun firing full burst at point-blank range might be 4+ for the first shot so it’s 5+ for the second, 6+ for the third, 7+ for the 4th etc up to 12+ for the 9th (or the max. size of the clip whichever comes first). Again, maybe stopping rolling after the first miss if it’s a single target, maybe not if there’s a clustered group.
Any time there’s a really weird result e.g. a character firing point blank with an SMG and rolling eight 3s then make it a jam instead.
This creates the realistic effect of automatic weapons being very deadly at close range but not much better than single-shot weapons at long range except…
pinning / suppressing could work off the same mechanic where near misses created the pin effect for example say the maximum roll to hit was set at 13 (auto miss) and a shot within 4 of hitting could have a chance of pinning then a 9+ would give a chance of pinning. In which case a burst of 3 from a machine gun at long range (needing a 10+ for the first shot, 14+ for the second and 18+ for the third would be turned by the maximum limit into three shots needing 10+, 13+, 13+ meaning 6+, 9+, 9+ for possible pins) would still have 3 chances to pin.
A roll of 2 could be a jam although that might be too much so maybe just a chance of a jam (this allows the opportunity for higher tech weapons to get a bonus other than more damage and maybe higher-skilled characters to get a bonus to their jam save to represent better maintenance).