Bullet trajectory is something else, again: Immediately upon firing LEVEL with the ground, the bullet flies at a slightly elevated upward angle from the bore centerline. If the bullet were fired perfectly level with the ground, gravity would immediately cause the bullet to begin falling back toward earth; however, because of initial sight adjustment and the (subsequent) upward angle of the barrel, the bullet will start to rise in an upward arc, instead.
As it flies the bullet will progressively rise along the arc of flight until it reaches an absolute zenith above the bore centerline. Thereafter the bullet will start to descend until it, once again, intersects with the bore centerline and, then, begins to fly below it on a downward arc of flight. The bullet will continue to fall below the bore axis until gravity finally pulls it back to earth. This phenomenon is known as the, ‘ballistic parabola’.
Here's a technical explanation of bullet trajectory:
The usual sight-in procedure is to, first, use a 25 yard target in order to simply, ‘find the paper’. Then go to 50 yards for the next initial indication of how the bullet is going to perform. Finally, 100 yard targets are used to identify the first (useful) POI.
With this conventional ballistic information in hand it is, now, possible to estimate a, ‘working’ Maximum Point Blank Range – Without having to resort to a computerized ballistic chart along with some really complicated ballistic equations.
As mentioned previously, this 30-06 rifle and 180 grain Spitzer soft-point bullet combination may be sighted in at three useful distances: 100, 200, and 300 yards. The information to do this correctly may be found in all of the better reloading manuals. For the purposes of this explanation I’ve chosen to use the Speer #13 Reloading Manual. The bullet information is found on page 298; and, the long range chart is located on page 711.
MAXIMUM POINT BLANK RANGE METHOD
Now comes the fun part. It might help to keep three important facts in mind:
First, the higher the ballistic coefficient is, the better the bullet is going to perform and fly.
Second, The greater the sight-in distance is, the more useful the, ‘Maximum Point Blank Range’ method is going to be. This means it is necessary to use a greater – rather than a lesser – sight-in distance in order to obtain a, ‘useful or working’ Maximum Point Blank Range.
Third, The VERTICAL DIMENSION of the target’s, ‘vital area’ must be accurately known.
It is generally agreed that, when viewed laterally, most deer have a vertical OVERALL vital area dimension of approximately 8”. Elk have a characteristic vital dimension of 12”; and, a typical human being has an overall vertical dimension of, at least, 16”.
So what is the, ‘Maximum Point Blank Range’ sight-in method really all about?
It is an old artillery term that originated in the early 1800’s, and originally applied to the behavior of cannon balls when they were fired at close range - level with the enemy. Over the years the concept has changed, and is presently applied to both artillery projectiles and small arms bullets.
A quick look at the conventional sight-in chart above reveals that the subject 180 grain Spitzer soft-point bullet will fly at a maximum height (or zenith) along a specific parabolic flight path. The upper limit is, therefore, fixed and, consequently, the EASIEST of the two vertical dimensions with which to work.
The simplest explanation of small arms, ‘Maximum Point Blank Range’ is to think of an invisible, ‘SIGHT TUBE’ through which the rifleman looks down range at his target. It extends over that distance from the end of the rifle muzzle all the way THROUGH the target.
The MAXIMUM DIAMETER of this, 'invisible sight tube' must correspond to two things:
1. The MAXIMUM UPPER and PRACTICAL LOWER limits of the bullet’s parabolic flight path must occur inside this, 'tube'. (In other words, you’ve got a vertical top and bottom limit inside which the bullet will travel and strike the target.)
2. The vertical dimension of the target's vital area must fit within this, 'tube'. (Got it?)
Now let’s extrapolate information from the above 300 yard sight-in chart:
1. The maximum upper vertical limit is, + 6.1”. (@ 200 yards)
2. The maximum lower vertical limit is, - 14.3”. (@ 400 yards)
3. Consequently, the largest conceivable vertical diameter of this, ‘invisible sight tube’ out to a distance of 400 yards is 20.4” from top to bottom.
These three parameters define the extreme limits of the vertical Maximum Point Blank Range for this particular bullet. A knowledgeable rifleman can, now, use this information to take a DEAD-CENTER Point-Of-Aim (POA) in order to put a bullet into the target’s vital area without having to guess at, ‘hold-over’ in all cases out to 300 yards; and, thereafter, only a small mental calculation (guesstimate) needs to be made for hits out to 400 yards.
Given these parameters, the only thing a rifleman has to take into consideration is a reasonable approximation of target distance and the particular vertical dimension of the subject target’s vital area.
The quicker marksmen who read this will recognize that the maximum lower limit is the most problematic of these two vertical dimensions.
Fortunately Coy Getman, one of Speer’s senior ballisticians, has assured me that this relationship is, more or less, proportional; so, in order to accurately determine the lower limit of a, ‘workable’ Maximum Point Blank Range all we have to do is, ‘guesstimate’ the amount of distance that the bullet will continue to drop between 300 and 400 yards.
Now here’s where I’ve been told to be careful: The old artillery definition would simply give the maximum PBR vertical limit as + 6.1”; but, this term does not really tell a rifleman where to hold in order to put the bullet into the subject vital area. It does, however, provide a useful starting point!
When holding a dead-center POA within the sighted-in range, (300 yards in this example) and for all targets with an 8” vertical impact area, it’s the effective LOWER LIMIT that a rifleman really needs to, ‘guesstimate’ before he takes the shot.
I’m going to make a real effort to avoid, ‘higher math’ and make exactly the same quick calculation, here, that I’d make in the field – Just as if I were the rifleman and this were my lateral shot on a standing deer:
OK, I’m carrying a 30-06 rifle loaded with a Speer 180 grain, soft-point bullet. (I only need one bullet because I never miss!)
The rifle has been previously sighted-in at 300 yards.
In the first scenario: It’s an easy level shot across a farmer’s field. The target is, about, 125 yards away. Aim dead-on and pull the trigger. The target will take the shot, about, 5” high.
(Coy Getman, at Speer, told me he uses this exact method while hunting; but, because he’s very familiar with his rifle and load, he knows from experience to, ever so slightly, lower his crosshairs on the target – NOT something for a newbie rifleman to attempt, I know!)
In the next scenario: It’s a difficult long shot across a small valley. The target is, more or less, level with your position at 350 yards; and, I have to go prone to take the sight picture. I know the target is past 300; but not so far as 400 yards. I, also, realize I’ve got a 14.3” drop to play with on an 8” vertical target zone.
This is a tricky shot in anyone’s book. Most riflemen would simply hold over, fire, and hope for the best. THAT is exactly what MPBR is designed to improve! This otherwise tough shot can, now, be made easier by using the Maximum Point Blank Range method of aiming.
My own thought process would be something like this: It’s given that 50 is half of 100. It’s known the total bullet drop is 14”. It’s, also, known that the subject vital area has an 8” vertical dimension. (Remember this stuff is, more or less, proportional.)
OK! You’ve got about a 7” drop to play with. All you have to do is hold your crosshairs a mere 3" above THE TOP of the subject's 8” vertical target area, and squeeze off the shot. You’ve got game!
Speer has warned me that, in their experience, many riflemen aren't able to learn how to do this without additional training and instruction; and, consequently, they don’t spend a lot of time on this subject in their manual and rely, instead, upon simple, ‘exterior ballistic tables’.
Well, ahhh, duhh! So, what else is new? This is, probably, true of every firing line I ever been on in my entire life. (But many of these riflemen were my competitors and, at the time, didn't have me to learn from.)
Horizontal bullet drift is NOT considered in this explanation; and, it is certainly not going to reveal itself inside 50 yards – That’s, ‘Why’ it is necessary to sight-in to, at least, 100 yards in order to more fully evaluate a bullet’s flight and spin characteristics. Right now, however, this is more than we need to discuss here.
The only thing it’s important to remember is to try NOT to sight-in on a very windy day. (If you do, then, make sure your bullet points-of-impact are on the upwind side of the target. By how much? Your guess is as good as mine.)
Be careful with your 25 yard shots, too! Close in, the angle of fire might cause the bullet to hit flat along the ground and, ‘skip’ around all over the place – Sometimes for miles!
Most of today’s reloading manuals contain information on sighting-in, complete with: ballistic coefficients, bullet trajectories, and, ‘WORKABLE PBR’ sighting information.