Locked weapons will become available from day 16.
Unlocking
When an alarm sounds, locked weapons will appear from the right side of the screen. Find and drag the appropriate key onto the weapon's lock to unlock it, then tap the weapon to select it. Whichever gun was tapped last is the active one.
Firing
After tapping a weapon to make it active, tap in the top part of the screen showing the birds-eye view of the border checkpoint. The gun will fire at the tapped location. Aim carefully; each weapon has a limited number of bullets.