Put some kind of inletting dye- lipstick, Prussian blue, etc. on the top edges of the pan, and then lower the frizzen to Mark the high spots. Remove the frizzen, and file down the high spots. Put it back, and repeat, until the gap is closed. Use a fine Draw file( teeth go in a single direction), and for the final finish, use a hard India stone so that the surface is polished. If no india stone, then put very fine Emery cloth around the file and use the backing of the file to polish the surface with the emery cloth. Put oil on the emery cloth to get an even finer finish. The finer you polish the pan, and the bottom of the frizzen, the harder it is for Powder Residue to stick to the surfaces, making cleaning your pan and frizzen a "snap" with a slightly dampened cleaning patch( spit works), and a swipe of the patch over the parts with your thumb.
Once you marry the frizzen to the pan, you will not have a gap to worry about again. Low End locks have these problems as a cost savings during production. As long as you understand why you can buy such a gun for so little money( compared to other guns with better locks) and have some basic Shop skills, you can make these locks and guns functional. :thumbsup: