I hate to disagree, but the gasses do not pass the bullets with lighter bullets. The problem is the gasses are still burning furiously when the 125 SJHP bullets, which are immensely effective BTW, are past the cylinder gap, and the hot flow of gasses can cut the forcing cone/top strap. If the only SJHP 125s you shoot are at bad guys, and maybe a few to check accuracy, you should have no worries. This is not an instant killed gun scenario.
I would not shoot a heavy diet of stout 357s of any ilk out of a 66. It is only my opinion, and others may differ, but it will get loose long before a Ruger just because it was originally designed as a 38 Special frame. Don't get me wrong, I really like S&W 19s, 13s, 65s, and 66s, and am thinking of getting one now, but I don't plan to launch barnburners out of them constantly. They are surperb shooting and feeling with 38s and milder 357s, and will last a loong, long time like that. Rugers are tougher, but S&Ws (at least the ones I have had) seem more refined. Like life, there are tradeoffs. My wife cannot bench pres 200 pounds, but I don't mind a bit.