Would you guy's feel comfortable hunting in the back country with an MSR Hubba Hubba tent and 0 degree bag. We will be hunting first rifle elk in north western Colorado? Do I need a 4 season shelter for mid October?
In my opinion, you should be fine. I ran that tent for years in first, second, third rifle seasons in CO. If you pick a decently well protected camp site from the wind and don’t encounter huge early snow I think you should have no problem.
In most cases hunting in CO that time of year you'll be much better served in a 3+ season tent like the MSR than a true 4 season tent. Lighter, probably larger, and better ventilation than a 4 season alternative (not to mention cheaper!). Pitch it in a reasonable location and guy it up well if you're exposed.