Got a gunsmith question. I am replacing the barrel on my old Smith & Wesson M&P .38 revolver because of a split on the bottom of the forcing cone. ( I have a thread titled "Do you see a problem?" on the revolver forum about this). I screwed the new barrel in as far as I could and still line up the sights and cylinder lock. The cylinder gap at this point is .004" to .005". Problem is I still have a gap between the cylinder boss and frame of about .012". Not enough room to make another turn. My question is can I shim the barrel or do I need to cut the boss on my lathe and then will have to cut the barrel gap and re-cut the forcing cone. Is shimming an acceptable method? I have found shims in multiple thicknesses on-line.