How can I . . .

[imemyself]

Is there any way to place a gun in a mission so its on the ground and you can pick it up, like its just been dropped from the action menu?

[Sui]

The object that makes dropping weapons possible is called: "weaponholder" (or "secondaryweaponholder" for secondary weapons. eg. LAW's, AT4's etc.)

You'll need to make one of those, and then add your weapons (and magazines) to it. So the code would look something like this:

wh = "weaponholder" createvehicle position
wh addmagazinecargo ["m16",1]; wh addweaponcargo ["m16",1]

So you would end up with a loaded M16 lying on the ground at position (position can be x,y,z co-ords, or (getpos object))