I think what Ruger did to compete was to try and create frame sizes that weren't being offered by S&W or Colt. I generally follow the in-between frames when comparing Ruger's to Smith's and it works. As for your Service Six it should fit comfortably in any K frame sized holster and use the same speedloaders.

SP101 = between J and K frame
Security/Service/Speed Six = between K and L frame
GP100 = between L and N frame
Redhawk = between N and X frame

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Ephesians 6 (Armor of God)