how to place weapons as static model

Weapons aren't compiled as static models, so they won't appear in-game when you place them as a prop_static (this also goes for cosmetics and class models). Place them as a prop_dynamic instead.

For future reference, the model browser has a tab with the prop's info (under the tab "Info"). There are 3 checkboxes there, with Physics, Static and Dynamic. Which ever box is ticked is the entity you can use the model with.