I've always wondered about this, which is why I've never invested in an IPSC shotgun (plus comps don't seem to be that common in Alberta).
So semi-autos are limited to 5 rounds, so the immediate thought is: get an 870 with a long tube. But the long tube itself can be fitted to a semi-auto, so doesn't that mean it is a prohibited device? Because I can't see a company making a tube saying that it only works with a pump, so therefore it is "designed" to be used with a semi-auto, ergo prohibited.
So how exactly do you extend the tube on a pump-action shotgun legally? And which pump-action shotguns come from the factory with a 5+ tube?
When you talk to gunsmiths or manufacturers anywhere else on the face of the planet they just can't wrap their heads around why you would want a pump rather than a semi-auto. Doesn't even occur to them that there's a market for large capacity pumps.
I did think about getting a competition shotgun from the US, like an X3 or an 11-87 customized or something, and then only fitting a long tube on it when I'm outside of Canada, but it just seemed like too much hassle for something I'd hardly use.