If you don't have any stockmaking stuff handy (like Inletting Black), you can use things like Permatex Prussian Blue,, Lipstick,, candle soot black, vasaline w/a small amt of acrylic artist color paint added,,ect.
Many stockers use different home brews as inletting color to coat the metal parts & leave a color impression on the wood indicating a high or contact spot.

Rub the coloring all over the underside of the bbls where the inlet of the forend covers.
Then put the forend back into place. Give the wood a gentle squeeze to press it firmly against the bbls.

Remove the forend and look for contact color on the wood.

Nice looking shotgun!