Off the S&W forum:

"I was recently watching one of the older Wilson Combat You Tube videos where Mas Ayoob was discussing his career and the duty guns he used. One Dept. he was on required to use the model 66 no dash as a duty gun. He said these guns where prone to locking-up due to heat expansion of the gas ring. And why the 66-1 moved the ring from the cylinder to the crane. And they were also prone to heating-up due to the steel they used (or something like that)." Link to video on the above thread.

https://smith-wessonforum.com/...ansion-gas-ring.html

I personally have a 66 no dash (4" bbl) and a 66-1 (2.5" bbl). I haven't shot either enough to find out if the above is true.