I didn't save my mission but I did make a new one. Just a couple of triggers. I attached it below. Just make a folder called takegun.intro (or whatever) and put the mission.sqs file in it.
I did this with an ak47. I did notice one interesting thing. If you are carrying a gun (like the m16) and then take the one on the ground, the trigger will not activate. If you drop the m16 and then take the ak, it works. If you substitute a secondary type weapon (rpg or law), then the script will trigger.
My theory on this is this. We are not actually creating a weapon. We are creating a "weapon holder". If we take the ak47 then the m16 is placed in the "weaponholder" and it doesn't "go anywhere" so its position does not change.
If we put a rpg in it, or we are empty handed, then no weapon is put back in the weapon holder. It then is deleted or disappears, thus it moves.
Basically, if we trade weapons the trigger won't fire but if we take the weapon without replacing one, it works.
Also, you will have to wait about 20 seconds after the weapon holder is created before you can take the weapon. An OFP issue.
Let me know if this helps.