Pauly your S&W M66 is a "K" Frame. Indyhunter summed it up well. The "J' frames are the small snubbies for the most part. The "K" frame is the medium size.The "L" frame took up the slack between the "K" and "N" frames.The "N" is the biggest frame size used for the larger calibers.Some years back if I'm not mistaking S&W made a "I" frame which was simular to the "J"frame.Not sure but I think the "I' frame is a little smaller then the "J "frame.Not sure how the letter designations came about but have been using them for several years. Soarkrebel can probably fill you in more about these Smith's. Ruger Redhawk

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