Reference import laws, but generally it's usually dependant on the mood of the CBSA agent on duty that day. There has always been this issue with CBSA not being very familiar with the exact laws of this and has always fallen into a "luck of the draw" scenario. I have managed to get multiple rifles access, while at the same time having things like mags, mock suppressors, receivers and even just outer barrels turned away. Your best bet is buying domestically or having a domestic retailer special order it for you for higher success
mk43, ump.45, p90, barret, m14, g33, k98, m4 tac, m4 swat, mp5 navy, vsr10, p226, m9.