Complete a game at least once on any difficulty level other than Very Easy, and save the game at the end. Begin a new game.
Guards are now much smarter and active in all modes now.
That means in Windows you should go to the Start menu, select Run, type in the path name of the game file. Completing each mode will unlock the next mode in the sequence. Technical-demonstration mode will then be unlocked. Use it to watch the CPU demonstrate its best time with the gun. Get weapons Hold backspace and press a number key like one, two, etc.