Hunting rifle accuracy increases in MOA size about 15%, sometimes more, for each 100 yard range band past the first one. Here's why.
Muzzle velocity spread; as the bullets don't all leave at the same speed, those leaving faster will drop less than slower ones down range. For each 100 yards of range, the bullets drop more.
Small variables in the bullet's BC, they all are not the same as the slight amount of unbalance makes them wobble in flight which increases their drag lowering their BC. A 3 percent spread in BC is common with hunting bullets; they're not balanced as perfect as match bullets.
Subtle wind currents between the rifle and target that we cannot see moves the bullet in all directions. Wind's not the same at all points in the bullet's trajectory. And wind above the line of sight is typically faster than in the line of sight and it varies with terrain.
Regarding accuracy needed to hit the 10 inch animal vitals at 400 yards. The very best competitors shooting prone, slung up in their best position, will add 1.5 MOA on average to whatever groups size their rifle and ammo tests at. So, if your ammo tests 1/2 MOA at 400 yards, in a field position, it's gonna shoot 2 MOA hand held by you; if you're one of the best wind condition readers and know how to adjust for it.
Best wishes getting your hunting rifle to shoot 1/2 MOA at 400 yards; no worse, ever. Few match rifles do that. Remember the accuracy your stuff shoots worst at is what you have to deal with afield.