Why a bullet rises above the line of aim and stays above it until it reaches a point for which the sights were set, and then falls below it, if not stopped; how much it does so, at various ranges and with various types of ammunition; how to determine the best point-blank for different kinds of hunting: these are matters that would require a good many pages to explain. (See the writer's Sporting Firearms, in the series of "Outing Handbooks").
In the big-game fields of the East and South, which generally are thickly timbered or bushy cut-over 2ands, it is seldom that one gets a shot at over 60 or 70 yards, unless he is in the mountains or on the margin of a lake or river. Even an old-fashioned rifle, using ammunition of, say 1,300 feet muzzle velocity per second, if sighted accurately for 50 yards, will not drop its bullet more than two inches at 75 yards, and at 25 yards you need only diaw a wee bit fine to cut off the head of a grouse or squirrel. Fifty yards, then, is a good elevation at which to set the rear sight of such a gun. A 30-30, or other rifle of the 2,000 feet M. V. class, shoots to this same "practical point-blank" up to 100 yards when sighted for 75 yards.
In other words, the hunter need make no allowance for distance, in aiming, up to these respective ranges, except when shooting at small game close by. This is why so many old hunters east of the Mississippi take little or no interest in guns more powerful than the kinds here mentioned. They don't feel the need of anything better. If they should unexpectedly have to take a long shot, and do it quickly, they simply draw a coarse bead, or In the West it is different. Much of the game country is open. Often you can't get close by stalking. Shots at 200 yards are common, and much longer ones can be made successfully by a well-trained marksman armed with a very accurate rifle that drives its bullet at a high and well-sustained velocity. I emphasize the words "accurate" and "well-sustained" because there are many rifles that are inaccurate beyond 100 or 150 yards, and that start their bullets swiftly but do not maintain a high velocity beyond short range. Their trajectory figures are illusory, because trajectory means only the average or mean height of bullet flight above line of fire at such and such intermediate distances. Take, for example, a .30-30 sighted for 200 yards. Its trajectory at 100 yards is given in the tables as 5.79 inches above line of fire; but, as a matter of fact, the shots vary so much at 100 yards that they may go anywhere from 3.40 to 8.40 inches high; at 250 yards, with same aim, they may drop anywhere from 2.25 to 14.75 inches below line of fire. Yet the .30-30 is considered a fairly accurate cartridge: there are others, with short, snub-nosed bullets, that shoot much worse.
Now, by contrast, let us consider a gun that shoots swift and true at all ranges, for instance one using our Springfield ammunition. I take the liberty of quoting from Captain Whelen the following table of such a rifle's actual performance, with 150-grain bullet, when sighted for 200 yards, and some of his comments thereon:
100 200 225 300
Above line of fire, in. ... 2.5 0 .... ....
Below line of fire, in....... 0 1.9 9.
Sight allowance, in......5 0 .12 .5
Above line of aim, in.... 2. 0 .... ....
Below line of aim, in........ 0 2.02 9.5
Mean vertical deviation, inches ................8 1.6 1.8 2.4
Greatest deviation from point of aim, with range unestunatsd, in. .. 2.8 1.6 3-82 11.9
"From this table, with the sights adjusted correctly for 200 yards and using the service ammunition, we can arrive at the following facts: Suppose one has not time to think of estimating the exact range, or has not the talent to do so. But he thinks his quarry is about 200 yards off. He fires. If the game was at 100 yards the greatest error he need expect is a hit 2.8 inches above the point aimed at. If the game was exactly at 200 yards then only the error of the rifle and ammunition need be counted on which is 1.6 inches either above or below. If again the game was at 225 yards the greatest deviation would be a hit 3.82 inches below the point aimed at. In other words, with the sights thus adjusted one would be sure to hit within the vital 8-inch disk at all ranges up to 225 yards, provided always of course that his sights were correctly aligned at the center of the disk at the instant of discharge. At no point during its flight, would the trajectory and the accuracy error, together, carry the bullet over three inches above the line of aim, and at 225 yards it would hit but 3.82 inches low. Should the range be over 225 yards, the visual angle subtended by the game, that is its appearance, will be so small that the hunter will not risk a snap shot but will instinctively proceed to take all those precautions necessary for a long range shot including a careful estimate of the range and wind direction and velocity and an accurate setting of the sights for those estimates.
"Thus, for all around work in hunting with the Springfield rifle, using the service cartridge, in order to attain the highest efficiency and the greatest chance for a properly placed hit, we should use three adjustments of the rear sight, as follows:
For small game at close range.......... 40 yards.
For apparently easy shots at large game 200 yards For apparently hard, long shots at any game ....................The estimated range".
The "vital 8-inch disk" refers to an expression I used in the book Guns, Ammunition, and Tackle, "Let us say that an 8-inch disk represents that part of a deer in which a bullet may be counted on to inflict a mortal wound; then the deer's killing zone would be that distance throughout which the trajectory of the bullet would cut an 8-inch disk. For open country, where long shots are the rule, the rifle may then be sighted for an extreme rise of 4 inches above line of aim, and the killing zone for deer will extend to that point where the descending bullet falls 4 inches below line of aim. Remember that line of aim or sight is different from line of fire (prolongation of axis of bore)." If the top of front sight stands one inch above axis of bore, then you subtract, from the midway trajectory one-half inch, and make proportional allowances at other points intermediate or beyond the range sighted for. In all targeting to determine point-blank you must aim exactly on the point to be hitónot at lower edge of a bullseye, but at its center.