A buddy of mine used to have a 30 carbine/308 adapter and had no issues with it causing problems with rifle chamber. I don't know if he bought it somewhere or had it made for him, but he had a huge amount of 30 carbine and no rifle to shoot it so he got this adapter somewhere and we shot a pile of 30 carbine from several different 308s.
I may just do something like this for myself but I think I would make up 5 of them, because they are such a pain to load and get the pistol case out and then reload..........might just as well use a muzzle stuffer.......but if a guy had a magazine full that's a whole nother matter.