I've heard that +P .357 should not be a steady diet for the 66.
But that with the new technology .38's can preform just as if not better. I've heard tell about the forcing cone will crack due to the under cut that allows the cylinder to clear/ close.
Am I overthinking?

I've heard a steady diet of 357 magnum can be harmful but 38 plus p should be fine.  Lighter load 38spl 125gr plus p or so can also be harmful

Most people just use standard .38 Special for practice with a cylinder or two of .357 Magnum to keep them used to it. Stick with 158gr .357 and you should have no problems.

This Model 19 has never seen anything but 158gr .357 Magnum from me, but I rarely shoot more than half a box in a session.

110-125  magnums can crack the K-frame forcing cone

140 & 158 magnums are ok

That is what s&w told me, years ago, when I got my model 19-4

. Attached File

You are horribly mixing up your terminology for one.
Any pressure level 38 special load is perfectly fine in a 19 or 66 and is probably going to wear the gun out at around a quarter million rounds give or take a hundred thousand
Light bullet 357 was found with heavy use ( example police agency range training guns firing hundreds of full house magnums weekly year round) to increase incidence of cracked forcing cones in these guns. No one even S&W knows what the limits are (? At what point volume wise does the risk increase?)
The general advice as noted by others is to avoid 125 grain and lighter magnums in k frames and you will be fine.
I come from a long line of heavy volume revolver shooters and can say from experience it takes tens of thousands of magnums to even begin to wear one of these guns out.
The generally accepted approach for almost all shooters is to shoot a few cylinders of magnums to understand where they hit compared to specials and then do 99% of future shooting with specials.

Thanks for all the replies!

My favorite load when I carried a model 66 back in the 70's and 80's was Winchester 145 grain Silvertip 357 Magnum.  The 145 grain Silvertip used flash retardent powder and the jacket went into the nose like semi auto pistol bullets.  The 145 grain Silvertip penetrated glass and steel car doors better than the lead nose jacket hollow point bullets.   There was a significant difference in muzzle flash shooting the 125 grain JHC 357 WW, RP or FC compared to the 145 grain Silvertip.  Recovery time with the Silvertip was the same or quicker than the hot 357 125 grain defense loads which were common in the 70's and 80's.   The deer I shot with the Silvertip were dispatched just as quickly has those I shot with the 125 grain JHP 357's.

If you can't or don't want to tolerate the magnum loads I prefer the 158 grain lead hollow point +P 38 Special "aka" FBI load.  My agency used to qualify with 148 grain HBWC or 150 grain lead SWC 38 Specials then it was the Officer's choice on duty load since the Department vary rarely provided duty ammunition.  This was a small department and when I was hired they gave me a mix of different 38 Special cartridges.  The Department never gave me another cartridge for duty use but required me to turn in 18 rounds when I left eleven years later.  Fortunately many agencies provide better equipment and training than they did back then.

Regardless the model 66 is a fine revolver but was replaced in LE with the L Frame 586/581 - 686/681 for Agencies which shot continually with magnum ammo.

The 125gr and 110 gr .357 Magnum loads are the ones you wanna not shoot much of.

I'd probably shoot more .38 through it than anything as it's just more pleasant.

Pretty good article on the topic.

http://www.gunblast.com/Butch_MagnumLoads.htm