I'd say within 500-600 yards because we are really guessing the dimensions of the animal that we subtend inside the moa or mil measurement.
When it comes to shooting rocks far away I use it to determine how big the rock I'm shooting at is. That'd be pretty close because you would use your ranged distance to figure that out; that would be less of a guessing.
Deer from back to brisket = 18" It subtends 3.5 moa. (18" x 95.51) / 3.4 = 505.6 yards
Or using Mills
Deer from back to brisket = 18" It subtends 1.0 mills.
(18" x 27.778) / 1.0 = 500 yards ------------------------------------------------------
Now let's say I range a rock at 1150 yards, and it's kind of square and it subtends 1.5 moa. Now for me to know its size I would do as follows:
Target Size = [(Distance) x (MOA reading)] / 95.51
its perfectly accurate if you can perfectly estimate the size of your target/game, and perfectly guage the MOA or mils in your reticle.
There in lies the only issue, estimation. i use a mil reticle and im not familiar with the MOA system but it works in the same fashion just different numbers.
I doubt i need to adress the difficulties in estimating the size of your target, all ill say is that it varies from hunter to hunter and their experience and the type of game you hunt and where, so its an open book there. If you can get a unobscured view of your target, broadside, and the target is large like an elk or bison, then knowing the size of your game or its shoulder height by glassing it can be reasonably accurate by an experienced hunter. The larger the animal, the larger (and easier) it is to measure in your FOV. optically ranging a rabbit @1000yds ACCURATELY on the otherhand, is almost impossible due to the small size, more on that later. The more accurate the estimation of the game, the more accurate the range will be.
A 10% error in size of animal, induces a 10% error in range, for that range. So 30yd error @ 300yds, no big deal. 100yd error @ 1000yds = missed shot.
Then the other issue compounding the first estimation, is guaging the number of mils/MOA the game occupies in your reticle. Again, a 10% error will induce an ADDITIONAL 10% error in your range, for that range. So rather than estimating 2mils, is it really 2.2mils? When trying to estimate the mils, try to do it to 1/10 mil or 1/4 MOA.
So if you estimate correctly within 5% animal size, and 10% mils/MOA in your reticle, youll still end up with 15% error in your range which is unacceptable for anything over 700yds, on large animals IMHO. Edit, assuming a highpower flat shooting rifle that is. due to the relative drops for the estimated distance compared with the actual distance and its point of impact on the game. If you induce more error ,which is highly likely, and shoot at smaller game, then the maximum ranges for using this become even less. Ethical shots inside 500m are reasonably easy if the shooter has substantial experience.
This is great info guys and helps a lot. It is confirming what I thought. I had just never spent the time to learn the technique and got to thinking I may be missing something. I will play with it on some targets of known size just to get a feel for it.