One of my kids has a brass frame Pietta 1851 .44cal that is at .015" cylinder gap. I never thought to measure it before, but checked today. I measured with the hammer at full cock and cylinder pulled back. In searching, I saw everything from .004 - .020" as being acceptable (.020" was debatable). I also verified that there were no cylinder imprints on the frame and the pistol has good lockup. It has never had heavy loads in it, always been around 18-20gr loads (I make their powder loads for them). Is this acceptable cylinder gap?