You don't. The BoomStick has 0.0 accuracy, that's one of its default attributes.
Quite simply, the gun is designed to be as inaccurate as is possible, solely due to the immense power of the gun if it were accurate. Similar to Sledge's Shotgun, if it had high accuracy, such as 80 or 70, then almost all of the projectiles fired would hit the hit-box of a targeted unit, dealing tremendous damage. If this were true, then killing Sledge and Baron Flynt would be VERY difficult.