You can actually cancel out of the burst-fire pattern just by un-scoping/releasing the aiming button. If you do this after your first shot you'll only fire a single shot. With this technique you can fire basically as fast as you can scope/unscope. Rather than burning multiple bullets you can fire a single shot.
The scoping/unscoping is less annoying than you'd think, since you'll usually be readusting your aim after a shot anyway. The time it takes to scope/unscope and shoot again is similar to the time it takes to recover from the muzzle climb of most non-Dahl sniper rifles.
You can even wait until after the first shot, letting two or more shots hit a target before canceling out; this is helpful for when you know your target won't drop after a single shot, but you still don't want to waste the whole burst. This is preferable to canceling and refiring on the same target as Dahl's burst fire shots are always much faster than the rifle would otherwise fire.
This works with all Dahl burst-fire guns (and works with burst-fire guns in Borderlands 1), but it's most useful with the Sniper Rifles.