Tack-Driving Upper, twist, length, bull-barrel QUESTION

[Train]

Gentlemen,
I shoot Tactical Rifle with my M4, and also hunt coyotes with it. My problem is that I cannot "reach out and touch" coyotes with any accuracy, and I cannot afford to buy another rifle.

SO I NEED ADVICE ON THE IDEAL UPPER FOR HUNTING. WHAT BARREL TWIST AND LENGTH SHOULD I CONSIDER FOR LONG RANGE ACCURACY?

My preferred ammo, when I can afford it, is 5.56 M855/XM855 62gr Green Tip FMJBT--and I'd like to use the same ammo for either upper.

For Tactical Rifle, I usually buy cheap 62gr because the targets are so close. I'm now using that same ammo hunting, since the Iraq war has driven 5.56 ammo prices out of my budget.

The range I need to reach out to is apprx 400 meters.
ANY SUGGESTIONS ON A TACK-DRIVING AR-TYPE UPPER I COULD DROP ONTO MY M4?