Not sure I understand your description, are you saying that the rib is 'swamped', meaning that it has a gap in the middle of 3/32" inch when you lay a straight edge on its top? If so, it was quite likely made that way, many guns were/are. I'm assuming you have the straight edge along side the front bead tight to the rib and not on top of the bead. Also some bbls give the appearance of having a slightly bowed look to them, but they are not. I'm going out on a bit of a limb here and say that it may be do to how they were 'struck' [filed]. You can hold the bbls up to the light and tell if they are bent or bowed by looking at the concentricity of the light circles refracted in them. If the multiple refractions are 'off' or not concentric, the bbls are indeed bowed or bent. That is not always bad. You have to shoot it to see where it is placing the patterns. In truth, you should do that with any shotgun, but many folks don't.
If I am reading your description correctly, I'm not sure there is a problem. However, if you think or feel that it is, then it greatly increases the probability that it will be. Not being 'smart' or smug here. Rather, trying to point out that confidence is a large part of successful shooting. If the rib remains a concern, you likely will not shoot the gun as well as one that you had more confidence in, that's all.
Hope something here helps you out.
