What game are you mapping for? If you are mapping for Half-Life, use
THIS fix made by me, so you can create a cycler_sprite to add floating models in your map without making Hammer crashes. If you're mapping for CS 1.6 / CS:CZ / DoD 1.3 you don't need the fix.
By the way, to make what you're trying to do, try to create a cycler_sprite with the weapon model you want to use (w_model I suppose), and then you can maybe try to use a game_player_equip. This entity allows you to equip a player with specific items/weapons when they join a multiplayer game or at some time during gameplay. So just add one in the map (I think you need a trigger_once which activates the game_player_equip, I don't remember correctly).
Try this out and tell us if it works!