Here’s my argument when guys start talking about HUGE boundaries for defining MPBR, or talking about 300+ yard zeroes...
Ok, let’s start with this premise - zero at 320, giving a trajectory peaking 5.7” above LOS. Seems reasonable, aim relatively low on the deer, and anywhere from 0-320yrds the POI won’t be high enough to miss anything important - or do you pretend.
First - let’s have a frank conversation about how that really works for establishing your 320 yard zero. Most guys don’t shoot small enough groups from hunting rifles to zero at 320, so they use some short range proxy and a calculated trajectory. So you propose shooting at 100, 4” high. So now we have to consider the integrity of our proxy zero - does 4” high at 100 really correspond to a 320 yard zero? Does it REALLY correspond to a peak of 5.7” as proposed? How sensitive is this to changes in density altitude? How sensitive is this to ammunition velocity changes? In under a minute with a ballistic app and one of my personal 30-06 loads, I tweaked my speed and density altitude and showed a 4” high at 100 yards plan might be anything from 300-325 yards zero, and might peak as high as 6.5”.
Equally, do you shoot well enough to establish a 4.0” offset at 100yrds? If you’re shooting 3 shots at 100 yards and pretending you have an accurate offset, you’re lying to yourself. A difference in 4.0 and 4.2” is 15 yards difference in POI at the long end.
Let’s also not pretend we’re able to perfectly identify the location of the animal’s heart at 200-300+ yards. Throw a 3-9x on your rifle with a 1/4moa crosshair, and a guy might be lucky to hold within +/-2”, even 3”. That’s before we even start talking about wobble zone size from common field positions.
Saving the most critical for last - most hunters who choose these easy rules of thumb are also the same hunters who can’t shoot for schidt. We’re likely talking about 1.5moa at best from a field position at 100 yards, and 2-3moa, at best, at 200-300. So at that “about 190 yards,” where your impact is “about 5.7” high,” we’re talking about HALF of your impacts hitting above 5.7”, some of which hitting 7.7” - which, again, is little more than a wild ass guess, since, as I described above, that “about 5.7” high” game might really be 6.5”, plus half of our group size, and we’re off POA by 8.5” - which has a 2-3” error margin in itself, so our “shot at the heart” might miss by over 10”.
I have never had so little respect for any game animal that I was ok with the notion of throwing a bullet, not knowing where within 10” my bullet will really land.
It’s not so difficult nor burdensome to establish a more responsible zero and establish a mapping strategy to establish a FAR more ethical congruence between our POA and POI. Grab a 100 yard zero, have your LRF at the ready to establish a range map, and know your drops. You’ll still fight POA placement and you’ll still fight gross group size, but at least you won’t be fudging a ~12” scribble box onto a deer’s vitals.
I find too many deadheads each season after rifle season, and I see too many “experienced hunters” in rifle classes who can’t shoot worth a damn to ignore all of these facts. Guys think they missed cleanly because they didn’t find blood, and they go about their day - while a high paunch shot deer runs away to rot from the inside out, and die slowly and painfully.
I’m not saying I have ANY issue with responsible, skilled, and practiced marksmen taking 300 yard shots on game, even twice that, but I’ve met seldom few folks who match that description who mess around with ridiculous strategies like the 320yrd zero proposed here.