The model 66 is a K frame, a medium size frame. (I also have one in a 4" barrel.) The N frame is the "large" frame such as the 44 and 41mag. J frame are some of the .32cal, 22cal, and 38specials (but are also in the K frame size). The believe the letters have more to do with the physical size of the frame, and not necessarily the caliber. Rather than calling it small, medium, large etc, the just use J,K/L, and N. Don't quote me on that though.

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