Ah yes, didn't say that in your post so I was like HUH?! Since when?
Yes, getting caught at the border by the US side of things with ammo would end up in a bad situation. But to be honest, I cross the border by land a few times a year, and on the return trip, I have never seen any US customs. I have only ever seen US customs people in the lines when heading into the US. But heading back to Canada, if any border police type people come up to your vehicle in the line, it's CDN. They have a chat and they may "randomly select" your vehicle for a search. So I'm not exactly sure how the US would enforce these rules with land border crossings. But not worth the risk anyways even with such low odds of getting caught.