I would consider an AR as having multiple rails. Receiver rail and handguard rail(s). I am sure that there are manufacturers that take rail spacing into account, and try to set them for a continuous rail. With the various upper and handrail makers as well as various mounting systems, is there really an industry wide expectation that the receiver and handguard should form one continuous rail with exact spacing? If there is actually a mil spec that specifies that? I don’t see it as very common to bridge the two rails?
