You should query him back and say it does not help. He has indicated that muzzle devices for what ever purpose do not count towards OAL but you are unable to find any supporting statements within the Criminal code to support that.
From the code itself
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“prohibited firearm”
« arme à feu prohibée »
“prohibited firearm” means
(a) a handgun that
(i) has a barrel equal to or less than 105 mm in length, or
(ii) is designed or adapted to discharge a 25 or 32 calibre cartridge,
but does not include any such handgun that is prescribed, where the handgun is for use in international sporting competitions governed by the rules of the International Shooting Union,
(b) a firearm that is adapted from a rifle or shotgun, whether by sawing, cutting or any other alteration, and that, as so adapted,
(i) is less than 660 mm in length, or
(ii) is 660 mm or greater in length and has a barrel less than 457 mm in length,
(c) an automatic firearm, whether or not it has been altered to discharge only one projectile with one pressure of the trigger, or
(d) any firearm that is prescribed to be a prohibited firearm;
“restricted firearm”
« arme à feu à autorisation restreinte »
“restricted firearm” means
(a) a handgun that is not a prohibited firearm,
(b) a firearm that
(i) is not a prohibited firearm,
(ii) has a barrel less than 470 mm in length, and
(iii) is capable of discharging centre-fire ammunition in a semi-automatic manner,
(c) a firearm that is designed or adapted to be fired when reduced to a length of less than 660 mm by folding, telescoping or otherwise, or
(d) a firearm of any other kind that is prescribed to be a restricted firearm;
Barrel length
(2) For the purposes of this Part, the length of a barrel of a firearm is
(a) in the case of a revolver, the distance from the muzzle of the barrel to the breach end immediately in front of the cylinder, and
(b) in any other case, the distance from the muzzle of the barrel to and including the chamber,
but does not include the length of any component, part or accessory including any component, part or accessory designed or intended to suppress the muzzle flash or reduce recoil.
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There is nothing in the above that indicates anything that is specifically excluded from OAL calculations where there is specific exclusions for barrel length. I would assert that anything that is mechanically connected to the firearm, I would put butt pads and flashhiders in that category, would count toward OAL as they are not specifically excluded.