Core · Unit · Rules

Breachers may enter a Building from any point on it—they do not require an Entrance. Breachers ignore Sited enemies and the Automated Sentries and Seal Effect Features when entering Zones. When a Squad with Breachers enters a Building, select one random enemy Occupying Squad. Each Breaching Unit inflicts X Small Arms hits on that Squad.

Source: Dropzone Commander Rulebook 3.02 · 3.02 · p. 44 · unit-special-rules