I'm always wary of doing those sort of 1/4" = 6mm type approximations; having operated in both imperial and metric, there's can be a lot of difficulty trying to mix the 2.
FWIW, I heard stories that the USA had a lot of aggravation trying to reverse engineer the German MG42 GPMG, which was designed and manufactured in mm and fractions thereof, when they tried doing it in 1/4, 1/8, 1/16, 1/32 and 1/64, 1/128th etc of an inch.
(Even when they finally got it to work, it ended up as the M60, so still not what you'd call a win really).
If the dimensions given are in inches, I'd be inclined to just go out and buy an engineers rule, marked in inches.