0. Try to find some armor and FN SCAR in Civilian.
1. Storage, since you will be running out of ammo soon.
2. Medical, when you run out of medikits.
3. Engineering. It works nowadays as Storage bis.
4. Security/Military/Main for extra fun and some chance of getting heavy weaponry.
0. FN Scar in Civilian quite rare :( Twice explore 1st floor completely and not find ANY of SMG (as a Scout and then Technician, I have only 9mm pistol)
1. If you have SMG (any) - go Storage. If none of that - go Medical first (you really need more medikits), and try pass to Engineering.
2. Medical for medikits and exploration bonus (not nessesary clean it out - just safely walk around)
3. Engineering. By this point you find some SMG anyway (or die x_X).
4. Try to explore 1st floor Sequrity - sometimes no locked doors here, or breakable glass rooms
Military always locked - IMO not wise explore 1st & 2nd floors (low keys/multitools at this point), go here from 3d.
Pistols quite useless even when mastered - you can't kill workers, elites, stalkers and other touch aliens with 9mm pistol. You can if you have unlimited health
Shotgun powerful but not good. No knockback as it do in DoomRL. Just a little more good than pistols. SMGs and pulse rifle are the key.
One shot 5.56 sniper rifle - useless, for big open space (like a Main tower) and 1-2 aliens at one time only.
Never find colt .44. May be it is a good pistol (I hope)
Shotgun can be found at Storage, but rare
Plasteel armor became common on level 4+ Military, Sequrity & Main towers
Twice visit level 6 of Main tower and find here Power Armor.