First 3 rounds in all bursts.
"You look a little off-colour there Roger. Are you sure you don't need a lie down?"
Toxic shells are definitely worth looking at when selecting a skill for your trooper. It can kill your enemy when your shot doesn't exactly kill your enemy from your first shot. Low-level troopers can be taken out by a single shell, while high-level ones will be able to survive until the poison wears off. However, it can be countered by sustained fire.
A major advantage of this ammunition is that even missed bullets poison their targets, this successfully counters defensive countermeasures (e.g. Dodger and Tuck 'n' Roll abilities). Lucky Charm's effect isn't capable to stop poisoning, either.
Lacking Gas Grenades, for poisoning enemy troopers en masse, you can utilise Fragmentation Grenades, which have an Area of Effect, delivering poison to closely packed troopers. The same effect can be achieved by using Shotguns or / automatic weaponry, but it's more risky to rely on.