You can have any length on an AR as it is already restricted.
You can cut any barrel down to 18.5" and its still only restricted.
However understand, that cutting a barrel over 18.5" to below 18.5" OR, and I stress the point OR cut a barrel that is already below 18.5" at all renders the barrel a prohibited device. The law is clear and what happens to those who break the laws pertaining to prohibited devices is clear.
Wishing it weren't so won't make it true. Some guy on the internet said it was OK is no defence. Ask any smith who would perform the work to provide a bill of work including authorization from the CFC to cut the barrel below 18.5". It will never happen with the current laws because it is illegal.
Oh sure, you'll find yahoos who'll do it but they're not going to sit with you in court, pay you're legal fees or ever get your guns back once they are conficated.
Just buy whatever barrel length you want, slap it on and save all the money you spend on lawyers to get yourself several new AR's in several different lengths and a great gun safe. Be smart, play safe...
And my 11.5 CMMG piston w/ a bull in 1/7 dances around an inch at a 100m. You'll be happy with it.