One Caliber Fits All

[Regolith]

If I, for some reason, was only able to have one gun, it'd be a 12 gauge shotgun. Extremely versatile, and capable of taking anything in North America.

 

[BusMaster007]

Get a RIFLE.

I'll choose the 7mm Remington Magnum.

It has light to 'almost heavy' bullet selection --- 100-gr. to 195-gr. ( Barnes Original, if you can find 'em ).
Varmints to Big Game then.

It flat kicks a$$.

Yes, head shots on small game.
Yes, deer at short to extreme range.
Uh, birds...? Well, that'd take practice!

Besides, it's the world's most popular magnum, so ammo won't be a problem...

 

[The Deer Hunter]

.308 Win

 

[Walkalong]

.308...it will do it all, especially in the hands of someone who can shoot 2" groups at 150 yds as you stated in your post. ( Head shots on rabbits )

 

[The Deer Hunter]

Well pretty much if you want to kill tiny game and big game a shotgun. But next best thing is .308 WIN.

Or .307 if you like lever actions.

 

[deadkenny]

Since this is the "Rifle Country" folder, I'll stick with a centrefire rifle suggestion - with my cartridge of choice for 'rabbit to deer' being .250 Savage.

 

[jkingrph]

If you reload, anything 6mm through 35 calibers. If not that restricts you considerably, most centerfire rifle ammo being two powerful for rabbits(leaving anything edible , if just shooting as varmits anything is ok).

Considering all that , probably the 32-20 or 32H&R Mag. up to one of the 357 mag. rifles/carbines. Load with 38 Special and not too much damage on a rabbit. All of those are marginal on deer, but with a close range well placed shot will do the job.