Ok then, let me put this another way.
From my understanding, NEC article 630 deals with the use of undersized wire based on the duty cycle of 'self-protecting' equipment (ie-welding) in certain circumstances (ie-equipment is dedicated to that circuit).
So if you put a 50amp receptacle/breaker in a structure (home or business) with 10/2 or 8/2 wire & upon sale of structure it is discovered by any inspector, in most states, you WILL NOT recieve a 'certificate of occupancy' for the prospective purchasers. So until corrected, say goodbye to that sale.
However, if fortune is on your side & nothing is discovered, pray every night that the new owners do not decide to plug in their motorhome, or some other 'appliance' that actually requires 50amps at greater then 10% 'duty cycle', into that 50amp 'NEC 630 compliant' receptacle/breaker. I think you (or your attorney) will have a difficult time explaining the nuances of NEC article 630 to a Judge & jury of peers, as you sit accused of 'gross negligence' regarding property loss, or God forbid, loss of life.
So for crying out loud, & piece of mind, use 6awg for a 50amp breaker/receptacle run.