As others have noted, the Ti-Scan S&W AirLites are going to be your lightest option. Unfortunately, they are priced accordingly. But trust me... the three ounces less they weigh make a world of difference when pocket carried.
I've had a 360PD for 11 years, and carry it daily. I'm amused by those who decry these things because of the horrible recoil, etc. Yep, the recoil with a full-house .357 is indeed horrible. But who said you have to shoot those? I carry +P .38s in mine. Make no mistake... even with the lightest target loads, these things are not fun to shoot. But a regular 15 ounce AirWeight (such as the 642) ain't no barrel of monkeys either.
The two Ruger LCRs I've handled did not impress me much. Both loosened up considerably after 100 rounds or so of target wadcutters. By that, I mean they developed some endshake (both yoke and cylinder) and the cylinder lock-up wasn't as positive as when they were new. Oh, they still work fine, and I have no reason to think they are about to fail; Ruger builds durable handguns. But polymer and revolvers is not a good combo, IMO. Witness that miserable polymer S&W "BodyGuard" revolver. Oh, wait... you did already...
I scored an early no-lock M-342 a while back; a delightful spare for my M-360PD. And I've missed out on two deals on M-340PD Ti-Scan guns, where the sellers apparently thought you had to shoot magnums, and they wanted no part of it.
Hopefully I'll run across another one someday; these guns carry like a dream.
Good luck with whatever you choose to get.
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