I'll reiterate the fact that out of a S&W 66 (that's a steel frame IIRC) you don't need lighter than standard 38 special start loads. I've found that I can do the flinch drill by myself just by leaving one chamber empty and then closing my eyes, spinning the cylinder, and closing the cylinder and not looking until I have the pistol up and aimed at the target.
One GREAT dry fire drill is to take a KNOWN and VERIFIED empty pistol (absolutely no live rounds should even be in the room with you) and a quarter. bring the pistol up to a shooting position and then balance the quarter (yes, other coins also work) on the front sight or on the top of the bbl. Practice squeezing the trigger ... you should be able to fire ANY pistol without the coin falling during the process. I only bring this up because I have seen some people that thought that they were flinching when it reality they were just pulling too hard on the trigger ... you won't flinch when you are dry-firing as there is no anticipation to cause it.
Remember that it is ALL mental, so you CAN fix it without any fancy and potentially dangerous reloads.
One note on the hazards of super light loads ... someone already mentioned the issue of getting a bullet stuck in the barrel. That is one hazard, but there is a phenomenon called "detonation" that can occur when there is too much empty space in a case ... as I understand it, when you have too little powder in a case, the powder ignites simultaneously instead of starting at the primer and building up pressure more slowly as designed. The result is a HUGE pressure spike that doesn't last too long, but its intensity is sufficient to cause structural failure of the chamber/barrel ... I know of a Springfield 1903 that fragged almost for sure a result of this (there was no way to double charge with the loads they were using, so a light charge was the only possible explanation).
Hope this helps.
Be safe,
Saands