Lowshot;
I wouldn't send it back. Usually pistols that fire lead bullets do better with a "step down" from the cylinder throat to the bore. The purpose of the forcing cone (the end of the barrel that almost touches the cylinder) is to ease the transition for the larger ball into the tighter barrel. This "squeezing" effect makes sure that the lead ball grips the rifling thoroughly, which leads to good accuracy.
Ideally, for cartridge revolvers that shoot cast lead bullets, you'll want a bullet .001 above the cylinder throat diameter. Further, you'll want a bore that's .001 or maybe .0005 below the cylinder throat diameter to get the desired "squeeze" effect.
Although you're pistol has quite a difference, I don't think that will matter much. Lead is very pliable and you should get good results.
My guess is that you'll have a great shooter.
One last thing. Are you sure you're measuring the slug from your bore correctly? I don't know, not having my Pietta handy, but how many lands and grooves are there? S&Ws are notoriously hard to measure since they have an odd number of lands/grooves. This means that very expensive V calipers are needed to measure the slug to get the true groove diameter. Pietta's may be the same way.
Good luck.
-John