My last wheelgun was a Webley MK 6 in unaltered .455 calibre. I used to scare everyone lobbing those huge 296 grain slugs downrange. A friend had the smaller .38-200 and discovered it had excessive headspace. A trip to a Birmingham trained gunsmith who restored fine Rigbys, Holland and Hollands, Westly-Richards etc. was most iluminating. He opened the action, laid the topstrap over a mandrel and gave it a smack with a mallet. The headspace was again perfect. Having a brit mechanic syncro my Minicooper S SU carbs with a brit pennie by ear fortunately prepared me for this insight.