39.9 mm COL? That's over CIP or SAAMI max (1.550"/39.37 mm) and with a 125 grain bullet would leave the case barely hanging onto it. If I use those numbers QuickLOAD predicts less than 8,000 psi, which would explain why so much N350 is left unburned.
If you want to run loads that light, I would change to VV N310. Maybe N320 at the slowest. The reason is that in a snubby you can get pretty extreme velocity variations with slower powders just because some powders don't burn completely before the base of the bullet clears the muzzle. That's not how it's supposed to be, but in a revolver, if the bullet base has cleared the barrel cylinder gap before the peak pressure has been reached, it can bleed enough pressure off that the powder needs extra barrel length to catch back up. 3" barrels are usually long enough, but 1⅞" barrels can have some trouble with it. A quick powder will peak before the gap and keep you out of trouble. It'll be cheaper to shoot, too.
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