Although the process sounds complicated, in reality it isn't...just time consuming. Fill out the paperwork correctly and you're good to go. My cousin moved back to Canada from Florida a couple of years ago. He'd spent about 12 years living in the USA and had built up quite a gun collection. Other than a Makarov, an AK-47 & a couple of 4" S&W wheel guns, the rest of his collection were easily imported. FYI, you don't have to pay registration fees or taxes/duty of them either, as they're classified as household goods.
With respect to your AR-15, you'll need to do a little more research on exporting that rifle from the US. The US State Dept. has become extremely anal and imposed many restrictions on what can and cannot be exported from the US. Best to check with your ATF. Since you are moving permanently to Canada, there may be some exemptions or different standards applied as the firearm in question isn't being shipped to a foreign national and you are the actual "end user."
As for your AR magazines, best advice is to pick up as many RRA LAR-15 mags as you can find. Make sure they are specifically marked for pistol use only. You will be shocked by just how much more we have to pay for things like mags, ammo, reloading components, etc... I would also look into bringing with you as much ammo as permitted under Canadian/US law. Ammo prices are crazy. Forget access to all that cheap surplus stuff you guys enjoy in the States. If you still want to try and bring in your 20-30 round AR mags, make sure it is physically impossible to insert anymore than 5 rounds. Drilling a hole and putting in a pop rivet is the most common way to do this. You can also use a punch and make a deep dent into the sides of the mag that would physically prevent the follower from passing below the five round mark. Others choose to weld/glue an internal block or dowel and/or glue/weld the floor plate permanently.
Also, as was said, all semi-auto centerfire rifle mags are limited to 5 rounds. This limitation doesn't include rimfire .22s. Handgun magazines are limited to 10 rounds max.
One thing you will like is the fact that because the AR-15 is a restricted firearm, there is no limitation on how short you can go with your barrel! No $200 tax stamp and hoops to jump through with the local LEO to own a shorty! The drawback is that you can only shoot it at an approved range; same goes for all restricteds. No gravel pit plinking with ARs or pistols in Canada! Since you're moving to Ontario, you'll need to find and join a gun club ASAP. Ontario's "regulations" are even more screwed up than the rest of the country and there is in a place a requirement for a person with restricted firearms to belong to an authorized gun club. You must then take the club's approved safety course and only then, will the club apply for an Authorization to Transport on your behalf. Crazy as it sounds, that's the agreement Ontario gun clubs made a number of years back. In most other provinces, obtaining an ATT is as simple as picking up the phone or fax.
Personally, I'd try to convince the missus to move to the States rather than you moving to Canada.
