The Bushmaster M4 is a semi-auto variant of the military version. A3 means it has a removable carry handle so you can mount optics.
So it's legal in Canada as a restricted firearm and can only be loaded and fired at an approved range.
There are many importers at the top of the page you could try and get one from but if you wanted to do the red-tape thing you could.
PS Barrels under 16" can not be exported from the US I believe. You're better off finding one here AFIK.