It is possible to take your airsoft guns safely and legally over the border. You need to have legally purchased them in Canada and be have receipts for each gun. Stop at the CBSA office prior to crossing the border and tell them what you're doing. They will give you a form to fill out, confirm the equipment and give you paperwork to take with you. On return you declare your gear and provide the paperwork with your passport.
I have successfully done this on 2 separate occasions. The initial stop took me about 20 minutes. Crossing back both times I did not get pulled over or searched.
Last trip was just over a week ago.
The Tier One Tactical Operator
Tippmann Arms M4A1
KWA USP .45, KJW P226, KJW hi-cappa