If you are talking about Hl1 I heard that you can spawn weapons with a monstermaker!
And if you can't do it with monstermakers you can fake it by making the gun invisable and place him on a high invisable platform made out of a func_wall_toggle. And when you will trigger the platform it will vanish and also create a env_render that will make the gun visable.(change the rendermode to normal)
To make a gun invisable turn off the
'smart edit' and add these values-
renderfx - 0
renderamt - 0
rendermode - 2
rendercolor - 0 0 0