You need to be more patient, CBSA can clear stuff in hours, or it could clear stuff in weeks. With a M4 front set in the box, you're looking at several days at the least.
As for whether a complete front set is legal to import (or perceived to be legal by the CBSA), I'd say that's definitely one of the items I'd personally be less willing to import myself. IIRC, people have successfully done it in the past, but it really is treading on a thin line as far as CBSA is concerned.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."