Pressing the fire button will fire a single group of bullets. If there aren't enough bullets in the magazine for a complete group of bullets, the group will consist of however many bullets are left in the magazine.
For example: The Gauss Compact Burst has a 2x Burst fire mode. When in this mode, pressing the fire button will fire a group of 2 bullets. If there is only 1 bullet left in the magazine before the weapon is fired in burst mode, the final group will consist of that 1 bullet, not the group of 2 usually fired.
Between each shot, a pump-action or bolt-action weapon requires a re-arm sequence to be performed. This is automatically performed, but slows down the fire-rate between each shot when compared to a semi-automatic weapon.