I have a digital grain scale, and after I make up an arrow as I want, I squeeze automotive silicone into the insert end, until it weighs what I want.
It goes in hard, but it goes. I then screw a tip in to clear the threads, remove the tip again, and leave the nock off to help it dry. (about two days) This took alot of playing around to find my desired weight.
It makes my arrows quieter (absorbs vibration) and increases FOC. I have not tried to remove the silicone on one yet, but I`m sure I could make a tool out of a small diameter steel rod to fit in the nock end to do so.