I have used the 2 part JB weld (goo consistency) and the Quick Steel (grey center & black outside).
Kiwi shoe polish as the non stick agent.

Depending on the rifle design, its easier to use one or the other. the goo makes much more of a mess but gives you more working time before it sets up. i found it was more prone to air bubbles as i played with it trying to fill all the spaces.

i will use quick steel again as just a preference.