As the owner of a Model 66-2 who has trouble resisting shooting at least a cylinder full of magnum loads on each range trip, I ask how many rounds would you estimate you fed your Model 66 in that "steady diet"? My Model 66 probably gets to the range a half-dozen times a year at best.
For clarity, I have a Model 19 not a Model 66.
At the time I cracked the forcing cone on my Model 19, I was shooting IHMSA silhouette competition so my gun saw alot of hot 357 Magnum loads. I'm afraid I cannot say how many hot rounds I put through the revolver but I was shooting for a year or so, maybe a bit longer before the forcing cone cracked. The range I belonged to had matches once a month during the warmer months and I practiced frequently in between.
After cracking the forcing cone on the Model 19 and while it was off at S&W being repaired, I bought a 10" 357 Magnum Contender for shooting silhouette. The Contender was eligible for production class at the time but, obviously, not revolver class. As an aside, I ultimately bought a Super 14 barrel chambered in 7mm International Rimmed for the Contender for unlimited class.
When the Model 19 was returned, I continued to enjoy shooting mostly warm 38 Special level ammunition loaded in 357 Magnum cases (158 gr SWC loaded to 900-950 fps). I'm not concerned shooting a cylinder or two of full power 357 Magnum ammunition in the Model 19 once in a while.
These days, I have several 38 Special K-frames revolvers that I enjoy shooting with 38 Special ammunition. If I want to play with more power, I have a couple L-frames to use. And if I want wrist snapping, 427 big block busting recoil, I drag out the 460 S&W Magnum.
Bottom line, heavy use of 357 Magnum loads in a Model 19 or Model 66 will probably result in a cracked forcing cone. Enjoying a cylinder or two of hot loads a few times a year will probably not hurt your revolver for many years.
I hope this helps with your questions.
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