Walkthrough Black Mesa: Source - Chp5 We've Got Hostiles
Channel: SuperAwesomeVidya
Date: 19/09/2012

Achievements:
"Gray Matter Propulsion" completed. (Started in the previous chapter)

You can start with the "Digital Wizardry" achievement in this chapter. Picking up a working sentry counts as "reprogramming" it. The screen on the back will turn blue and the sentry starts shooting at enemies. 
(I will demonstrate this achievement in Chapter 8 "On A Rail")