Convert S&W M66 (357) to .40 S&W or 10mm ?


June 22, 2010, 10:10 PM
Can this be done ?
Has anybody heard of such a thing ?

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Chuck Perry
June 22, 2010, 10:17 PM
Nope, never heard of it done. Not sure the 66 is up to the challenge. Maybe 40, but I would say definitely no on the 10mm. There is a guy, Dave Clements I believe, that converts Ruger GP-100's over to 10mm. Nice finished product, but the conversion starts out around $600. For the cost of the GP and the conversion you could have a new S&W 610 and a trigger job.

June 23, 2010, 12:55 PM

The K-Frame size Model 19 & 66 already have part of the barrel shank milled away to clear the cylinder gas ring.

That makes the forcing cone very thin, and has contributed to the K-Frame .357 Magnums being discontinued due to cracked forcing cones.

Making the bore .400" would only make the problem much worse then it is with a .357 hole..

In addition, reaming the .357's .380" chambers to the .40/10mm size of 425" would leave the cylinder walls & webs between the chambers paper thin.


June 23, 2010, 01:07 PM
Just FYI...S&W already makes a 10mm revolver, the N-framed Model 610, from which you can also shoot .40S&W.

Although somewhat rare, they also made the L-framed .40S&W Model 646.

June 23, 2010, 01:18 PM
What in the world for?

June 23, 2010, 05:03 PM
Moon Clips.

Ala Dan
June 23, 2010, 05:06 PM

And why would you want to ruin a perfectly good S&W model 66 in the first
place? :scrutiny: :eek: :banghead:

June 23, 2010, 05:10 PM
I really can't see the Smith "K" frame handling a conversion like that.

June 23, 2010, 06:06 PM
I can't see how it could be done either but over at the Greybeardoutdoors forum (??) sometime back there was someone who had posted a conversion to ,40 S&W I think to a Model 19 done around the Spokane area by a gunsmith. He claimed that if the loads were kept "light" the gun had no problems. I wonder if it was the same outfit that made the .44 Sp Ruger Sixes?

Lucky Derby
June 23, 2010, 06:12 PM
S&W did it with the L frame (646) not the K. The K will not handle it. I wonder though if a 646 could be bored to accept 10mm. I have seen a very nicely done M28 converted to 10mm. Smith also made the 610 on the N frame.
If I were going to attempt such a thing and for whatever reason did not want a gun as large as an N frame, I would most likely look at a GP100.

June 23, 2010, 06:30 PM
From a simple materials engineering standpoint it would not seem to be safe.

June 23, 2010, 08:10 PM
The K frame S&W is not up to the task. It was designed as a lightweight revolver for .38 Spl that can fire .357 mag. If you open the cylinder you'll notice the bottom of the barrel isn't perfectly round, this is the weak spot of the gun.

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