I've loaded 41 magnum probably more than any other handgun cartridge and definitely longer than any other but never tried 3N37. I never used Blue Dot much in handguns. As was mentioned, 2400 is a great powder. Very versatile. 4227 is another good powder for heavier loads. It can take downloading better than H110. It would be considered a slow powder in handguns. I use a lot of HS6 and Unique for mid range loads.
H110 is a great powder but a one trick pony. It doesn't like down loading at all. I tried back in my handgun handloading infancy and got a few hang fires. I certainly wouldn't recommend it for a mid range load like you are seeking.
I checked Quick Loads for 3N37. It appears to be a mid range powder very close to HS6 in burn rate. Using a Lee 210 grin SWC and 10 grains of 3N37 QL predicts a velocity of 1022 (from a six inch barrel) and a pressure of 15,705, well within the 41 Magnums safe working parameters. Going to 12 grains takes it to 1179 fps and 22,686 pressure. You can use the Quick Load recommendation if you like. I wouldn't be afraid to try that load in either of my 41's and if it were me loading with what you have, I'd use HS6 data since 3N37 is slightly slower than HS6, (but very close) and keep my velocity in the 1000 fps ballpark.
In your original post you said he has an S&W 686. Unless I've missed something, I have not ever seen any factory 686s in 41 Magnum. Is it a 657?
