From the FAQ
Okay, so I canít import guns. What about parts and accessories?
Yes. The good news is that most parts and accessories, including most magazines, can be successfully imported by individuals without requiring a BFL. Examples of things you CANNOT import:
AEG magazines with fake cartridges (i.e. P90, G36, SG 552 mags)
Frames, receivers, and metal bodies
Suppressors and silencers
Prohibited-length outer handgun barrels (i.e. USP Compact, Glock 26)
Hand and 40mm grenades that look real
Examples of things you CAN import:
Magazines for AEGs and GBBs without fake cartridges
Outer barrels for most guns
Inner barrels of any type
Scopes, red dot sights, iron sights, lasers
Rail systems and scope mounts
Springs, bushings, gears, pistons, or other gearbox parts
Batteries, BBs, loading tools
Helmets, goggles, body armor, tactical vests, camouflage
Note that CBSA inspectors have discretion to detain goods of questionable legality, and although it shows up on the above list it MAY be held if you draw an inspector that is particularly ignorant or of foul demeanor. That said, I have yet to hear of any seizures of the above items.
Prohibited-length handgun barrels may or may not be stopped. Most outer barrels are not dimensioned internally to even appear capable of firing a real bullet, and faux rifling marks typically extend only a few millimeters down the bore. However CBSA inspectors are not firearms experts and if they think there's a potential of it being usable in a real gun, they'll seize it rather than risk their jobs. Best not to take a chance on it.
YANHCHAN'S AIRSMITHING: AEG repair/Tune up/Upgrades V2/V3 mechboxes, rewiring/reconnecting.
Ask not what your country can do for you; ask what you can do for your country ~John F. Kennedy