I've seen a lot of noobs complain about this problem, leading them to take uneccessary shortcuts or downgrading the gun. So I made this video. I found that any other method can lead to the risk of damaging knex, while this method leaves knex perfectly safe.
Yes, I'm modding my TR a bit.
*EDIT* If you are trying to pound a gray rod in with this method, use a black rod to push the gray rod in its slots.