The mag extension is not a prohibited device. When installed and not pinned the firearm becomes prohibited. But if you install and pin the mag extension to load only 5 rounds it is perfectly legal and remain a non-restricted or restricted depending on OAL.
Not true. A mag designed for a semi auto center fire long gun (except the garand and a other one I don't remember) that holds more than 5 rounds is a prohibited device. It doesn't affect the firearm's classification. Your sks stays non restricted even when fitted with a 10 rounds mag. But the mag is illegal.
Definitions
84. (1) In this Part,
prohibited device” means
(d) a cartridge magazine that is prescribed to be a
prohibited device, or
91. (1) Subject to subsections (4) and (5), every person commits an offence who possesses a firearm without being the holder of
(a) a licence under which the person may possess it; and
(b) a registration certificate for the firearm.
Unauthorized possession of prohibited weapon or restricted weapon
(2) Subject to subsection (4), every person commits an offence who possesses a prohibited weapon, a restricted weapon,
a prohibited device, other than a replica firearm, or any prohibited ammunition, without being the holder of a licence under which the person may possess it.
http://www.canlii.org/en/ca/laws/stat/rsc-1985-c-c-46/latest/rsc-1985-c-c-46.html
But without the spring in it, the mag can no longer feed a gun with ammo. SO it'S not a firearm magazine
You can legally possess a 30 rounds mag designed for a long gun semi auto centerfire, as long as it is dismantled.
It doesn't have to make sense......