You need to send in your BATF forms for your firearms you want to bring over there temporarily, they will send you a reply whether they allow it or not, bring those replies with you at the birder, tell the customs you're bringing in firearms and that you have the forms, they'll ask you to go in, they check the firearms and the authorizations from BATF. You're good to go.
Coming back into Canada tell the customs guy you have your own firearms coming back in, they'll check your license, your registrations if they are restricted, the firearms, probably the mags, you're good to go.
You might feel a little awkward the first time, but after you know the process you'll be a whole lot more comfortable. You just need to plan your trips to the USA a lot in advance so that you can get the BATF forms and the replies and all.
No Chinese firearms are allowed in, a bunch of other firearms too that make no sense, but essentially if your barrels are more than 16 inch and the firearms are american you're good to go.