So I did some more research... it seems that, before I go to Canada, I have to fill out a Form 4457 at a CBP office, which is a "registration for personal effects taken abroad". Upon returning to Canada I just have to declare my firearm and show them the 4457.
1: Go to CBP office, register firearm on Form 4457
2: Declare firearm at Canadian border, show my PAL
3: Declare my firearm coming back to the US and show the the 4457
Is that it? It seems much simpler than I thought it'd be.
Edit: this is what I've found for NON-RESTRICTED firearms only. I can't find anything on bringing restricted items across apart from "call the RCMP".