I kept the single action on my bobbed hammer. I use it for stupid things like hundred yard plinking. Dumb idea for SD because once you cock it, you have to shoot it. Yes I am aware that isn't exactly true, but anyone that thinks this is a solid idea doesn't know how to do that.
I shot John Farnam's Defensive Handgun course with a Chief's Special drawn from the pocket. Probably shot about a hundred rounds. I did put my thumb under the hammer when drawing and didn't seem to really have a problem in terms of speed and accuracy. I shot pretty much everything except for maybe 20 shots from 50 feet away single action but everything else was double action.
The second day I had the pachmayr compac grips which were actually too big for my hand on my airweight 638. I shot that about 100 rounds for the second day mostly from 25 ft and closer.
I did the qualification test at about 25 feet at an 8 inch by 6 inch reactive target and made it to the student or second level pin. It did take me about 10 attempts though. I think I would have done better with the Chiefs Special or even my 3 inch 36 - 1. Again however during the test at least I didn't have the time to cock the 638 into single action. However I think the heavier pull with the too big grips under time constraints with a reactive target did affect my accuracy.
Speaking of the cocking John did a demonstration for us using my Chiefs Special of how the revolver was designed from 100 years ago with with a safety feature that would not allow the firing pin to strike the primer if there wasn't pressure on the trigger. Later on I tried this test with my 638 and to my dismay when I tried to decock it while releasing the trigger and dropping the hammer the gun did go off. And yes it was at the range and I had it pointed down range in a safe direction. It may be that the first gunsmith who did the action job took too much off. I'll have to get that checked out. There is a method for safely decocking with an exposed hammer but as John pointed out is harder to do with the shrouded hammer which is why he recommended never cocking the hammer on the 638. Makes sense to a point but anyway I'm going to get the 638 checked out.
Okay examined the decocking on my 36 and 638 the other night. Did about 30 repetitions with each. Mechanically they both decock fine. However with the shrouded hammer 638 is much easier to hang up the hammer part way and then force the firing pin to engage when you try to correct the error. Or at the range under a bit of stress trying to use fine motor skills to pull the trigger whole riding the hammer nub down.
I was working an old clothes assignment as a young cop carrying my Colt Cobra under a tee shirt. Grabbed the gun while arresting a street robber/mugger and the hammer got snagged in my shirt. Fumbled and got it out, but had the range guys bobb the hammer after that. Still have that little Colt.