In the safe opening scene, Kevin Dillon's pistol alternates between the slide being forward and the slide being locked back- hence, empty.
When Bruce Willis and Kevin Dillon are arming themselves, Kevin looks down and states there's 8 shotgun rounds in the box- but the box he's looking into clearly reads "CO2" containers- commonly used to power BB-guns.
At 78 minutes, when Rosa is forcing Shane to open the safe, the exact same shot of Kevin Dillon saying "Come on" while pointing a gun is used twice. This is made particularly obvious by how the way Rosa is holding the gun immediately changes in the subsequent shot after the second "Come on".