The Saga Of The Uphill-Downhill Shot, Part I
With that being said I still had a ballistic issue that demanded resolution. In order to better understand the problem I dug out all of my old notes from Art Pejsa et al and began my own research project that would have made Isaac Newton do a double back flip. When discussing this scientific issue with several of my industry acquaintances, the subject of temperature, altitude, barometric pressure and the like invariably came up. One of these enlightened souls was convinced that the disparity in point of impact was due to the shooter canting the barrel differently between the sight in shots and the shots in the field.
Leanne Woslum and great granddaughter Briella. These two are very pleased that we got this guy after several attempts.
A quick review of these dynamics is at this point no doubt in order. For those readers who are ballistically astute the following may be a bit ho hum but believe me the vast majority of the weekend hunters and even a few of the so-called tactical experts are grossly misinformed on these topics.
ALTITUDE DIFFERENTIAL – An altitude increase of 10,000 feet raises the point of impact for most bullets one inch at 300 yards. At 500 yards this becomes a correction of 5.5 inches.
TEMPERATURE DIFFERENTIAL – A temperature decrease of 50 degrees lowers the point of impact 2.5 inches at 500 yards. An increase of 106 degrees equals a ballistic change of one MOA.
GRAVITATIONAL INFLUENCE – Other than wind and the velocity of the projectile, this is the most influential of all the ballistic dynamics and the most often misunderstood.
The basic premise is that any object will fall from any height at a particular rate according to its mass. It matters not if that object is dropped from a platform or if it is propelled from a rifle barrel.
Hypothetically let’s say that you were to stand on an 18 foot platform and drop a 168 grain .308 bullet and that it took this bullet one second to drop the 18 feet. If one were to then load this same bullet in a cartridge that would propel it at 3000 fps from a rifle, it would still fall at that same rate i.e. 18 feet in one second. Gravity is gravity and you can’t fool Mother Nature (or should we say you can’t fool quantum physics).
The above hypothesis assumes that the barrel is oriented perfectly parallel with the earth’s surface. If one were to elevate or lower the muzzle of the rifle by 60 degrees, gravity would then no longer be working at 90 degrees to the bullet flight and the bullet would not drop as far in that given length of time. Now this is where the wicket gets a bit sticky. Most of the $20 uphill/downhill doping gizmos available today will accurately measure the angle of the shot and will just as accurately advise you what percentage factor should be applied in order to properly resolve the shot solution. All is good to this point but here is where they go awry.
Hypothesis – We have a steep downhill shot and the reading from our manual type inclinometer indicates it to be 60 degrees from the horizontal and that we should apply a factor of 50% in order to correctly impact the target. The 60 degrees is correct, and the 50% adjustment is correct. The instructions then tell you to apply the 50% factor to the actual distance to the target. Physicists will now tell us this application is completely 100% wrong. This is due to the manner in which we adjust our sights in order to compensate for bullet drop. At an angle of 60 degrees the component of gravitational force is one half of its vertical value, not one half of the distance to the target.
If we were to apply the above process to a hypothetical range situation it could read: distance to target = 300 yards, the slope is 60 degrees the percentage factor is 50. 50% of 300 = 150 therefore shoot the target as if it was 150 yards. This would be a big mistake.
The correct application of the slope dope is: The bullet drop from bore line to 300 yards is about 23 inches depending on the exact weight. In order to compensate for this bullet drop we must lower the reticle of the scope (so as to raise the barrel) the MOA equivalent to this correction factor. This equals about seven MOA.
In order to put this into a real life scenario, let’s use my .280 Ackley, shooting a 150 grain TSX bullet at 2900 fps, and using my Leupold 4.5 to 14 with ? minute clicks for sighting equipment. Our objective is to hit a 400 yard target on a 60 degree slope.
CORRECT APPLICATION – The scope adjustment required on the flat is 6.75 MOA or 27 clicks. Apply the 50% factor to this (.50 X 6.75) = 3.37 MOA. Actual come up clicks required 13.
INCORRECT APPLICATION – 50% of the distance (.50 X 400) = 200 yards, which is only 6 clicks up. This seven click difference between the two methods is equal to a seven inch disparity at point of impact.
Steve Adelmann, retired U. S. Army Sniper, and PS columnist, and I have discussed this issue extensively. Steve being of inquisitive mind and having dealt with this sort of problem on a daily basis, was more than willing to run some real life up/down slope problems through his various programs. Alas the results of these tests further confuse the issue. As Sergeant Major Adelman said in his most recent message to me: “The only issue these various programs agree on is when shooting uphill or downhill, your bullet will impact higher than it would if shooting on level ground.”
As far as practical application for a sniper in theater is concerned, Steve says that most snipers have worked out their own solutions to these problems and have the dope permanently affixed to their buttstock.
Using a standard U.S. sniper .308 load sighted dead on at 100 yards, Steve provided data from 10 different service manuals and computer programs including the U.S. Army’s, Marine Corp’s, and National Law Enforcement manual. An example of these data is as follows:
RESOURCE 150 yd. level hold 300 yd. 60 deg hold
Sierra Infinity Up .6 MOA Up 1.5 MOA
Trag MP Up .6 MOA Down .2 MOA
Special Forces Divide the
Manual measured range by 1/2
Quite obviously there is a huge difference of opinion on the solution to this ballistic issue.
After this heady discussion where does this leave me with the mystery of shooting too high on downhill targets? I haven’t the slightest clue!! But there is great hope for me yet. The technical folks at Leupold have agreed to come to Yellow Wolf and do an extensive test on all these issues. There are indeed gremlins about and we are going to ferret them out. Stay tuned.
Join the discussion of this article HERE at the Article Discussion Forum.
<Previous | Home
All times are GMT -5. The time now is 09:56 AM.