The Leadwall has a magazine of 12 shots but each pull of the trigger consumes 3 shots, which means it can only fire 4 times before it requires a reload. This low magazine capacity is complemented with a fairly high reload time. The weapon deals high damage at short range, firing 3 spread projectiles that bounce off of walls, shredding Titan armor and eliminating pilots in one shot. However, the shots will dissipate after a certain distance, making it useless for long range combat.
Try to get in front of Titans to hit their critical spots, and then Phase Dash behind them while reloading.
The Leadwall shares many similarities to the Triple Threat from the original Titanfall, using a similar magazine drum, but having 3 barrels instead of 1. Even the concept art depicts the weapon with the name Triple Threat.