When installed you have to go to:

mods>update>update.rpf>common>data

find dlclist.xml

hit "Edit" in the top

right-click dlclist.xml

scroll to the bottom

make new line

copy and paste this line

<Item>dlcpacks:\servicepistol\</Item>

!!!! NOTICE !!!! NOTICE !!!! NOTICE !!!!

YOU HAVE TO USE YOUR OWN GUN MODEL SINCE I CANT UPLOAD THE FILE WITH A MODEL IN!

To install a weapon model, head to 

servicepistol>x64>models>cdimages>weapons.rpf

drag and drop your desired weapon

You shouldnt use anything other than a pistol, since the slot is made for a pistol (Glock of basically any kind)