A break-action scatter shotgun designed for close-quarters hallway fighting in GLMZ's compressed urban corridors. The BA-4 uses a traditional break-action mechanism machined from Ablonite-KR ceramic composites, making it virtually immune to electromagnetic interference and impossible to disable with standard EMP grenades. The barrel is short — just 28 centimeters — and the bore is wide enough to accept a variety of improvised loads in addition to its standard flechette shells.

Crucible Industries markets the Doorknocker as a breaching tool rather than a combat weapon, a legal distinction that allows it to be sold at Tier 2 without heavy weapons licensing. In practice, every corridor fighter and close-quarters operative in GLMZ knows the BA-4 for what it is: a devastatingly simple weapon that turns tight spaces into kill zones. The break-action mechanism means a maximum of two shots before reload, which has given rise to the street saying 'two knocks and you're done.'

The weapon's Ablonite-KR receiver has an unexpected secondary benefit — the ceramic composite is extremely resistant to forensic trace analysis, making it difficult for investigators to match fired projectiles to a specific weapon. Crucible claims this is incidental to the material choice. Nobody believes them.