If a vehicle fails to stop at a base entrance, the gate guard hits the button that raises a barrier some distance past the entrance. Then they go for a weapon, if necessary. Post 9/11, that will be something with more power than a pistol.[1]

The barrier solves an old problem - what action to take when a vehicle fails to stop at a checkpoint. Most of the time, it's drunks or minor crazies. Lethal force is excessive and car chases on base are dangerous. Barriers became standard for DoD installations around 2005.

[1] https://www.youtube.com/watch?v=iW79PLFuUGw

Many police carry a secondary handgun in case they lose their primary or whatever.