At least out here, it is illegal to shoot from a vehicle. Pretty sure a tractor would be considered a "vehicle".
I am not a fan of 222/223/224 caliber for deer. But, a buddy of mine made two clean shots a couple years ago; two shots, two deer. Heck, I have never seen more than one buck in California. Much less two. He had two tags. Two bucks came over a ridge. Bang, bang. Two deer, two shots in just a few seconds, and two deer dead. About 100 yards with a 222 Remington, in a bolt action.
In regards to the AR-15, I think it is unfair to to just count them out. The AR platform has really evolved. In some area's it is legal to hunt with a 0.224" bullet ie 222, 223. The other thing, is over just the last few years, other cartridges have evolved for that platform. The 30AR, 6.8 Grendel, 6.5SPC, and some others. The Grendel is even used in 1000 yard match.
Although it is not my platform of choice, I see no difference in hunting with an AR, as compared to a Browning BAR, or Remington 740/742. Those are nice rifles, available in a number of calibers, even 223.
I would not though, myself, go hunt with an AR carbine 11.5" barrel with suppressor and collapsing stock. Nor, more than the legal 5 round magazine. I'm not big in to dumping a full magazine in to a deer either.
But, an AR in 30AR or 6.5SPC, 20" full float barrel with a flattop and a scope, and a 5 round mag. Well, that would match anything you could find in 243 or 308. For accuracy and range.
For now though, I'll stick with my old Savage 99 lever. Best deer gun ever made. .300 Savage, a fine, fine cartridge(and forerunner of the 308).
First question I have, would be, is it legal to make a 400 yard shot at a deer with a firearm from a tractor? Surely you guys wouldn't be talking about it if this weren't the case. After all, this is a tractor forumn...
Just for the record, I am a gunnut and people shooting deer with AR-15 in .223 aren't my favorite kind of hunters.