I spoke to two S&W CS guys about b/c gap after receiving a "repaired" 642 which had the frame replaced, and a new b/c gap of .011" on the left side and .005"-.0055" on the right.
S&W CS guy #1 said as long as it's anywhere between .004" and .011" it's good. Iow, the gap can vary up to .007" and it's good, according to him. I thanked him and figured I'd call a few hours later and speak to someone more CS oriented.
S&W CS guy #2 said the b/c gap range was .004"-.012", BUT it should be somewhat consistent. He emailed a FedEx label to me, and the 642 still at S&W.
The used Charter Arms .38 has a b/c gap of .007" on the left and .005"-.0055" on the right. I'm very pleased with that.
Another thing I've started checking is whether or not lead bullets will pass through the cylinder throats (because of leading issues). 158gr LSWC .38 Special bullets will pass through the throats on the Charter Arms. The throats in the original cylinder on the 642 were too small to allow a lead .380 bullet to pass, and that's about .002" smaller than the .38 (which should pass).
I also find the sights on the CA snub MUCH better than those on the 642 and LCR, especially the rear sight.