I have no experience with the 7.62x39 barrels, but my heavy contour .223 has digested a diet of military 5.56 ammo almost exclusively since purchased in 1994 delivering excellent accuracy with ZERO difficulties.
That 'no milsurp' disclaimer from the company is, from a practical perspective for the user, merely over-cautious hedging IMO.
From the practical perspective for H&R however, when you consider the company's history of bankruptcy resulting from product liability suits, it's the only sensible advice to offer.