I was thinking of something more along the lines of this:
Captain has pass code 1
Head 1 has pass code 2
Head 2 has pass code 3
Head 3 has pass code 4
In order for the doors to open, the captain and head 1,2, or 3 must input their pass codes within 1 minute of each other. The door CANNOT be emagged, hacked, or otherwise fucked with. The doors are powered by batteries however, and the captain/heads can swipe their card and replace the battery. The store room would probably also be protected by turrets inside of it which would make getting weapons hard. I'm thinking of a diagram sorta like this:
R R R R R R R
R R R R R R R
R R T T T R R
R R T L T R R
R R T T TR R
R R R D R R R
R R R P R R R
R R R D R R R
R is Reinforced walls
T is Turrets
D are the doors
P is the turret "panel" (can be turned off using the captains pass code so a militia/trusted people can also get weapons.)
L is the locker with the guns (requires a head/captain ID swipe.)
The back/side walls should probably be surrounded by space, and if not, they should be triple layered. Also, the turrets should automatically not shoot at anyone that can be recognized as a head or the captain. In order to prevent grief, they must have an original head/captain card, OR if they had their card taken they must be able to be identified (have no headgear on so instead of saying Unknown it says their name).
If a Greifer wants to get guns, they must:
A: break into EVA, steal a space suit, break into the chemist lab, make thermite, go into space, break through 2 R walls, not get shot by 8 turrets set to kill (cannot be set to stun), have a head/captain ID to get into the locker, and steal the guns.
B: break into the chemists lab, make thermite, break into the bridge (where else would you keep a gun locker?), break through 3 R walls, not get shot at by 8 turrets, have a head/captain ID to get into the locker, and steal the guns. (If they don't make thermite I'm pretty sure a captain/head would find them, and they would be in a whoooole lotta trouble)
This is NOT completely grief proof, but if it is grief proof, then it's also traitor proof. I believe the traitor should be able to get in, but only after doing extensive planning, and even then, there is a lot of risk involved.