not entirely accurate. as legend has it, GIs in the south pacific were able to fire 7.7 japanese ammo out of their 30-06s when supplies ran short. of course one should never put too much stock in such tails but just last winter there was a member here that had bought a type 99 arisaka and had issue with his 7.7 brass blowing out and it turns out that his gun had been rechambered to 30-06 by a previous owner and was never marked.The idea that a rifle chambered for 308/7.62 Nato can fire a 7.62 x 39 probably originated in Vietnam with all the stories about "They could fire ours but we couldn't fire theirs", and too many forget that while a longer straight walled case can fire shorter case rounds-.357 and 38 Special/Long Colt, 44 Magnum and 44 Special/Russian, the same is not true of rifle cases with shoulders, even if the case in question is shorter and the bullet diameter is the same.
I had a pair of adapters like that. I don't know if they were made by MCcase or not but they came from sportsmans guide. I had one that was 303 brit to 30 tokarev and one that was 7.62x54R to 30 tokarev. extraction sucked but other than that they work pretty good.Kinds sounds like a crap shoot. Scares me!