I would suggest that you have someone familiar with these revolvers take a look at it before contacting S&W. The underlug is not meant to contact the frame firmly and a small gap of 0.005 to 0.015 inch between the lug and frame is normal. If that's what you've noticed, it isn't anything to fuss about.
I suspect that if the lug actually did contact the frame, the effect on accuracy would be terrible because free floating barrels do NOT respond well to intermittent contact. Basically, without that gap the lug would vibrate against the frame and this could cause the barrel to NOT "ring" true. If you want to see the effect of intermittent contact on accuracy, try resting the barrel on a sandbag while shooting from a bench. What will happen is that your groups will double or triple in size. BTW, I've been there done that and resting these barrels on a sandbag results it groups that are NOT pretty.
Another possibility is that the barrel may not be mounted perfectly square in alignment with the top rib on the frame. This has become such a frequent complaint that I'm starting to suspect that S&W is intentionally "clocking" the barrels in order to have them shoot zeroed with the adjustable sight set with the rear blade centered for the windage adjustment. Clocking the barrel is a technique that dates back to the 1870's for zeroing fixed sights and they may be reviving this technique. The best way to test if the barrel is properly set for the sights is to center the rear sight blade and shoot it in single action from a benchrest at 25 yards. If the resulting group is centered within 2 inches of the sighting point, it's not worth bothering with because you'll have plenty of adjustment for windage on the rear sight. Note, best technique for benchrest shooting is to rest your hands on a sandbag and shoot carefully with the barrel free of any contact. Lay off on caffiene, have a decent meal of proteans and complex carbohydrates about 2 hours before the range session and with a bit of practice you should be able to see 2 to 3 inch groups.
One additional note, I've noticed that 38 spl. and 357 Magnums require different windage settings for me, why I'm not sure of but I suspect that it's a matter of recoil management. Basically, the heavier recoil of the Magnums causes me to "throw" the bullet a bit differently than the lighter recoiling 38. They also change a lot in elevation but that's to be expected when changing the velocity and energy levels this much.
Finally, when you have a question about issues such as this, PICTURES are quite helpful, it elminates so much of the guessing.
I am shipping it back with a printed photo of the problem,if it doesnt get properly fixed its getting traded in. 
