Alright this is how gun adding works.
You first need a table which can change the item (Redduck Bat) into something else. Once you hit buy it purchases the item you changed to gun code to.
The Bat's code is:1111 (no its not)
I will change it to 2222 (M4A1)
I hit buy then BOOM! I got a M4 instead of the bat.
Not all of the codes work but this is how the basic of gun adding.