I've been looking at what's available there. Not a huge selection available in Canada. I see a bit more power becoming available with something like this 220gr .300BLK cartridge, priced at around $1.25+tax per round, delivering about 488fpe (my Winchester JHP 147gr at 1,100fps delivering 81% of that muzzle energy) which doesn't seem such a useful increase considering the cost of a .300BLK rifle... and then of course everything in the class is full size rifles, no takedowns...
https://www.reliablegun.com/en/fede...blackout-220gr-otm-subsonic-20rds-box-1000fps
Seems the .35 stuff is generally made to travel around 2,000fps and hotter, so perhaps not the best ballistically when it comes to cutting velocity in half? And the mass involved doesn't seem to offer anything over .300BLK in terms of muzzle energy when loaded subsonic.
I'm leaning more towards thinking about a shorter non-restricted 12ga shotgun with either a rifled barrel or 'rifled' slugs. A 1oz slug at 1,000fps gives 970fpe, with similar holdovers to 9mm or .22lr. Not so sure about precision shot placement however, which is my major point of hesitation with a shotgun, ESPECIALLY if trying to keep it small-ish for convenience in backpacking. This page has some good information on subsonic hunting:
https://www.americanrifleman.org/articles/2015/6/19/subsonic-shotshells/
While of course it's not absolutely essential that I go subsonic, it's a strong preference. I'd rather stick with just the one set of custom earplugs, not layering up with external hearing protection. It's not just a matter of being excessively sensitive to loud noises, which I am, but that my hearing is central in my profession, as musicians depend upon my ability to adjust the sound of their violins, violas, cellos and doublebasses with considerable sensitivity. So going hunting and being somewhat able to hear what's going on around me is at odds with my career. Subsonic ammunition seems the obvious compromise. And since I don't relish the prospect of hauling a full-length shotgun around in forests... a shorty seems to join the list, though not quite so imperative.
Sorry, going far off the 9mm question.