Use oil or lube like Kano Kroil down the barrel and use a wooden or fiberglass or carbon fiber dowel (from a crafts or hardware store or a thick wooden BBQ skewer that barrel fits the diameter of the barrel) to ram it out muzzle loader style pound it in with a hammer if you must. Don't use any rod made of metal. Use a towel so you don't damage the gun.
LISTEN at 2:10 -
https://youtu.be/YoG43rmkFF8