You can do this three ways, I may however forget the other two on the way to describing the first.
Make your locker, with a door etc. Right behind the door (don't have it in any way poking through the door) create a trigger_once and in it's target field/line enter akmm.
Now place a cycler_sprite inside the locker, this will be the most awkward task. In the sprite line enter: models/ak47.mdl, now you can either, have teh gun lying on the floor of teh locker, or open up teh model in an editing program (ms3d) and change it's origins so that teh gun faces with teh barrel pointing 'up'.
Or you could try tweaking the Black Circles with wee lil' white lines in the middle (also known as Pitch, Yaw and Roll) in teh cycler_sprite's properties and position the model how you want it.
Name teh cycler_sprite: akd
Right next to the locker (it could be all the way across the map, but for tidiness put it next to the locker) place a multi_manager
Open it up and name it akmm, turn Off Smart Edit and click on add, then enter this:
Key: akd (short for AK Disappear)
Value: 1.5 (this is 1.5 seconds, adjust it as neccasary)
add again, and:
Key: akg (short for AK Give)
Value: 1.5
Place a env_render entity, name it akd; open it's properties and set it's render mode to solid, and it's fx amount to 1.
Place a game_playerequip entity, open it up and name it: akg
Scroll down through it's list of guns and select: ak47
Turn Off Smart Edit and click add:
Key: ammo_7.62nato
Value: 4 (gives four clips)
I've been doing other things while writing this so if any of it seems sketchy just ask and I will clarify.