There's 25.4 MM to an inch, but simply doing the math doesn't always work. For instance, if we multiply 25.4 by .308, the calculator spits out 7.823 - - - - -not the 7.62 that we usually associate with ".30 caliber". Inch/MM conversions are a SWAG at best!
Jerry
7.62 / 25.4 = 0.3 the .300"
bore size associated with 30 cal
