I have and old Crescent that's a tad loose. A new dollar bill will just fit between the barrels and breech. Internet search came up with a thickness of .0043" for US currency, a soda can is .0038", made a shim from a Coke can last evening... just a smidge loose. My place of employment has a machine shop, an email to the shop foreman has a small .005 shim stock in the interoffice mail as I type this.
Craig


Ask not for whom the dog barks, it barks for thee...
NRA Life Member