Except that the restricted/non-restricted barrel length threshold (for semi-auto centre fires) is 18.5 inches, not 18 inches. 18 inches is the minimum length you may cut a barrel down to before it becomes prohibited.
Beyond that, the problem here lies with ill-informed officers, loads of non-restricted AR look-a-likes, and the continuing use of bad terminology like "all handguns are restricted, along with some military type rifles".
The XCR, ACR, SU-16, Swiss Arms rifles, SR-22 and other tactical .22s, etc all vaguely resemble AR-15s, and failing that they would strike an officer as a "military type rifle" that would prompt him to investigate further. Other non-traditional firearms such as the Tavor, RFB and so on may run into the same issues.
Just like with your 10 round pistol mags, it is wise to be pro-active when it comes to this type of thing. Carry the appropriate legal documents with you, including your gun's FRT number. I keep them in my glovebox and a copy in each gun case. It's pretty cheap insurance. We shouldn't have to, of course. But I'd rather present the officer with a piece of paper that reassures him than have my guns siezed only for them to discover I was right 3 weeks later.
Until our modern guns become more commonplace, this is our situation. Remember to be courteous when dealing with officers, and don't forget to offer them the opportunity to try the gun out. Shake hands and leave a favourable impression.