The evil NRA's meme, repeated by Rubio, of "can not define firearms" enough to regulate is BS.
Canada has no problem. Drill down to the regulations to see how it is done in detail. As far as I know firearms are firearms, Canada, US or any other nation.
http://www.rcmp-grc.gc.ca/cfp-pcaf/faq/index-eng.htm#a3
Q3. What are the different classes of firearms?
A3. There are three classes of firearms: non-restricted, restricted and prohibited.
Non-restricted firearms are ordinary rifles and shotguns, other than those referred to below.
Restricted firearms include:
handguns that are not prohibited;
semi-automatic, centre-fire rifles and shotguns with a barrel shorter than 470 mm;
rifles and shotguns that can be fired when their overall length has been reduced by folding, telescoping or other means to less than 660 mm; and
firearms restricted by Criminal Code Regulations - DETAILED definitions and LIST.
See, you can make a LIST...as long as you like and modifiable every time the vile NRA and its GUN factories come up with another name or modification.
http://laws-lois.justice.gc.ca/eng/regulations/SOR-98-462/page-2.html#h-4
Prohibited firearms include:
handguns with a barrel length of 105 mm or less and handguns that discharge .25 or .32 calibre ammunition, except for a few specific ones used in International Shooting Union competitions;
rifles and shotguns that have been altered by sawing or other means so that their barrel length is less than 457 mm or their overall length is less than 660 mm;
full automatics;
converted automatics, namely full automatics that have been altered so that they fire only one projectile when the trigger is squeezed; and
firearms prohibited by Criminal Code Regulations.
Q4. What is considered to be a firearm for purposes of the Firearms Act and for offences related to the Firearms Act in the Criminal Code?
A4. As set out in section 2 of the Criminal Code, "firearm" means:
a barrelled weapon from which any shot, bullet or other projectile can be discharged and that is capable of causing serious bodily injury or death to a person, and includes
any frame or receiver of such a barrelled weapon, as well as
anything that can be adapted for use as a firearm.