If you've completed the museum level, you already know there is a firing range with guns and melee weapons.

Go to the firing range entrance:

Wait! There are 2 gates. Look above. If you cross the green line the first one will close and the second one will open. Therefore, we will block the first gate from closing like this:

Just use a trash can or two so you can crouch under. Now cross the green line and the gate won't shut completely:

That's it, now go throw everything into the reclamation bin near the gift shop.

Minipop103: You can also just put all the weapons next to the door and kill yourself to open it from the other side

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