Except for some Corbon loads.......and maybe some "botique" 357's any factory 357 will not hurt a pre lock 66. Current production 357 ammunition is not loaded as hot as it was in days past.

I have fired a case of Remington 125 grain 357 Golden Sabre through my K-frame magnums, including my 66 snubs, without ill affect. I personally prefer the Speer Gold Dot 158 grain 357 due to its ballistics and effect on the recieving end of the round.

Shooting 357's is not for everybody though -noise/flash/recoil........or optimum in every situation.......such as indoors in crowds. These days there are some very effective factory 38+P loads. These are easy to shoot and very accurate with impressive performance. I like the Speer 135 grain 38+P short barrel ammunition and frequently carry it in my 2.5" 66-2.......and sometimes in my 4 inch 66's. It is low flash and I do not find the recoil objectionable. It is a VERY accurate load in my experience as well.

In any case, get a round for your 66 that you can shoot accurately with fast accurate follow up shots as well. Then go practice! Regards 18DAI