As stated above it is on the same frame as the OM .357s introduced in '55. Old Models being 3 screw, no transfer-bar medium frame revolvers. As we know Ruger builds their revolvers like a tank, so the .357 was built to 'last' and take anything you can load into it within reason. Then Ruger put the .357 in the large frame when the NM BHs were introduced in '73 which was 'way way' over built and dropped the OM medium frame BH. Anyway point is your medium frame New Vaquero is 'made' for the .357. Go out and shoot it with no worry. Enjoy! Oh and as a side note in Ruger lingo, all Vaqueros are New Models, but there is an original Vaquero (large frame) and the New Vaquero (medium frame).