Agree with both the gentlemen, above. I always set my Dan Wesson (that gun's gap is adjustable) up for 0.003"-0.004" B/C gap for target shooting, even though the gauge that came with it was 0.006". I used 0.006" for .357 loads or for double-action practice where the pace and heat picked up over bullseye match pace. In other words, the bigger number was for reliability in combat type shooting, the smaller one was to help low pressure target loads maintain pressure uniformity fired at a modest pace. I never had a malfunction from the narrower gap, but it never went very long without cleaning in its target capacity.
When I bought my Ruger Redhawk years ago, the shop had three of them they had just received sitting in the display case. I had not gone in expecting to buy anything, but at the sight of them, temptation took hold. I had no tools with me, but I checked cylinder play as Harry describes, by feel. I checked all three with the thumb drag test*. I held them all up to a light and made sure the barrel cylinder gaps were parallel-sided and even. I hope it goes without saying that I checked to be sure there was no ammunition in the guns when I first picked them up, but a double check before the next step was in order: I put a strip of white paper between the cylinder and recoil plate and held it so some light fell on the paper. I looked down the bore while cocking the hammer. I checked to see which gun's chambers were best centered in the barrel when in battery. One was perfect, and that's the one I bought.
Below is an early 50 yard group from that Redhawk fired off bags using a low power Burris scope. I don't know why I didn't date it? It is circa 1986. The American Eagle ammunition would have been the first box I bought for it. One chamber in the gun is a little too tight and always throws the one flier you see in the group below. I've considered reaming the cylinder, but I don't want to mess with the success of the first five chambers, and buying a specially ground reamer to match the sixth to the other five is a bit spendthrift for my taste. I just don't use the sixth chamber.
Nick
*Drag thumb lightly on cylinder while cocking the hammer to be sure the hand rotates the chambers enough for the bolt to snap in and lock up every chamber. One of the three failed this last test on two chambers, so it was weeded out.