Okay, 2 minutes wasn't much, here's my concept for the idea:
; Plystire made this script, you WILL give him credit
; (\_/) |
; /- - |
;<| - |>O Or Ruthless Samurai bunny will KILL you! >:3
; (_|_)
desc = Pickup Weapon and disable further pickups (Drop weapon with G) (Uses Global Var 67)
:always:globalvar=67
:state=0:state=1
;/////Load any needed HUDs in state 0
:state=1,plrdistwithin=60,varequal=0:playertake,setvar=1,state=2
;/////Modify below line to show "SWAP" hud when player already has a weapon
:state=1,plrdistwithin=60,varnotequal=0:
:state=2,scancodekeypressed=34:playerdrop,setvar=0
:state=3,plrdistfurther=65:state=1
That oughta do it. I didn't put in any HUDs for this, but I put in comments to help you out with that.
[EDIT]
Unfortunately, even though this will work, I would not suggest making enemies drop weapons, because the player will be able to pick those up and have more than one weapon.
Those are also weapons that the player may not be able to drop.
[/EDIT]
The one and only,