I will try to explain the terminology used by S&W. In the begining, Smith & Wesson designed the "K-22 (meaning K frame, and .22 caliber) as the "Combat Masterpiece". The
model was introduced with a 6" barrel. A few years later
and at the demand of public out-cry, they decided to bring
upon the scene a 4" barrel model, designated the model 18.
Then with newly found knowledge that stainless steel could
be used in handguns (in 1965), Smith & Wesson introduced the
same basic design with it's model designation being 617. The
figure 6 is for stainless steel construction; and the 17 is
for the famed "Combat Masterpiece". Likewise, the 317 is of
Titanium construction, and is Smith & Wesson's newest "kid
on the block". All this is probably confusing to anyone new
to the "GunWorld"; but I hope this helps some.
All of these weapons chamber the .22 Long Rifle cartridge.
Best Wishes,
Ala Dan, Life Member N.R.A.