Burst is a Weapon Stat that affects how many rounds can be fired in a row without having to take Aim again. The only thing that limits bursts — regardless of how many rounds are fired — is magazine Capacity. Technically, all weaponry has a basic burst of 1, while automatic weapons can go more. If capacity allows, semi-automatic (1) Weapons can fire more than 1 round per burst, but only with specific Upgrades. Only automatic weaponry can fire in bursts by default. Naturally, this includes Assault Weapons (3~4) and Machine Guns (5~8). Therefore, they are subject to Spread, a variation of Area of Effect.
There are weapons able to fire two rounds per trigger pull, but such a feature is not treated as a burst, being a special skill instead. Thus, the magnitude of ammunition spray can be extended even further.