It's one of those cases where the law is on your side, but an officer might not see it that way, which is unfortunate but understandable. You probably couldn't get in trouble for it in the end, but if a cop stopped you and you had an m14 with a bunch of what appeared to be 20-round mags for it, he probably wouldn't buy your explanation at the time. It's such a badly written law, it's hard not to sound ridiculous when explaining it to someone!
If it was me, I'd get an AIA rifle - as well as an m14 - and store the AIA mags in a case with the AIA rifle, separately from the m14. I'd even have the AIA mags engraved as such. That way, in the unlikely event that an officer wants to inspect your gear, he'll see the 20 rounders together with the bolt gun and not think anything of it. Keep your ducks in a row, and save yourself the hassle.