I’m relatively new to this gun stuff. Before the 2000, I didn’t have money for firearms, and after I bought my first round of firearms, I didn’t have money to purchase guns for a long time.
I was forced by good comments to go do math and then visualize it. Thank you, gnuplot.
In the graph above, the first line is purple and represents the Point of Aim through the scope 1.5 inches above the bore.
The second line, green, is a simple constant acceleration of gravity in an ideal environment. I.e. bull crap. It is there so that I can have a verification stage.
The next line is the results from the Hornady ballistic calculator app. The values input are BC of 0.12, muzzle velocity of 1050, diameter of 0.22inches, weight of 40 grains. These are the numbers from the CCI subsonic 22LR ammunition.
From looking at the constant acceleration and the G1 calculations, you can see there is a difference. The bullet is losing velocity in the G1 graph and drops more than 8 inches at around the 67 yard mark. The constant acceleration has that happening at around 72 yards.
Next, I “zeroed” the rifle/scope combination at 25 yards. This requires that there be an initial upward component to the path of the bullet. I did not do the math correctly to modify the initial horizontal velocity based on stealing some of that for the upward motion.
The orange line is again constant acceleration. The curve is tangent at 25 yards. Depending on initial velocity, this might not be a tangent, but instead pass upwards through the PoA curve. The M193 out of an AR15 platform, zeroed at 25 yards, continues upwards for a significant distance.
The yellow dots are the Hornady G1 calculations with an initial upward velocity added.
What does all of this mean? It means I’m a visual person and should have spent the time graphing this stuff, so I could actually see what I’m doing.
Thank you for the feedback.
I’m getting better.
At issue is learning the easy things, but not understanding them.
Before I put the new scope on the rifle, I was hitting my target(s) at random ranges consistently. I put the new scope on the rifle, zeroed it, was, very, happy with the groups I was getting.
For many reasons, I didn’t have an opportunity to press the trigger for a while. My partner wasn’t here to help with the meat processing, the weather was too yucky to flesh the hide. I just didn’t get a chance.
I did glass a number of targets over the week, but no shots taken.
Then the other day, everything aligned, and I was good to go for a shot. I waited until he presented a beautiful side view, sighted in to put a round through his eye… slowly pressed the trigger, bang. And he took off. Clean miss.
Hmm, what did I do wrong? The next day I had another opportunity. Again I waited for a good presentation, lined up to take him through the eye. Pressed the trigger.
That damn critter didn’t even flinch.
Why was I missing at these ranges. It didn’t make any sense to me.
My mind started working the problem.
The answer came to me, bullet drop!
I was wrong.
Over the distances I was shooing, the time of flight was less than 0.08s. Using that old school math, we find that the distance a bullet drops in a given time is 0.5at^2 a=32ft/s^2. Fumble fingering the calculator, that gives me a drop of 1.5 inches.
Shit. That’s more than enough to miss that rodent’s entire head.
Add to all of this that because the rifle is sighted at a known distance, it is only at that distance and one other where the point of aim and the point of impact are at the same level.
At 25 yards, the point of impact will be either 3.5 or 3 inches below the perfectly parallel to bore scope. I say 3.5 or 3 because I do not remember whether the scope is 2 inches above bore or only 1.5.
This means that to have the point of impact intersect the point of aim at 25 yards, I have to point the scope down 3.0-3.5 inches.
But as we move the target closer or further from the muzzle, the amount of drop changes. This makes a difference. There is also that difference in angles. With the scope pointing down to get the rifle bore to point slightly up, there will be a period of time when the bullet is low compared to the point of aim.
After that time, the bullet will be high, compared to the point of aim.
Then we add even more stupid to the equations.
I got the new scope because I wanted the hash marks on the reticle to help me with figuring offsets.
The scope is lower cost, so it is a second focal plane reticle. This means that the distance between hash marks stays the same regardless of the magnification.
When the manual says that a hash is 1 MOA, they mean it is 1 MOA at 9x, not at 4x, where I keep the scope.
This means I need to know the magnification to use the hash marks to measure with.
I’m going to head back to the range and do some actual measuring. Then I’m going to use a ballistic calculator to have a better idea of wtf I’m doing and where that bullet is going to be relative to point of aim.
If things go well, I might even be able to figure all of this out, without blowing up my brain.
Once all of this is figured out, I get to repeat it with the Marlin in 30-30.
Comments
4 responses to “It is all about the math, or how come I keep missing? UPDATED”
“At 25 yards, the point of impact will be either 3.5 or 3 inches below the perfectly parallel to bore scope. I say 3.5 or 3 because I do not remember whether the scope is 2 inches above bore or only 1.5.” – Your bullet does not drop an inch at 25 yards just to get a second wind and pull up to zero at 100 yards.
With rifle calibers you have a maximum offset at the barrel that tapers towards your “zero” (lets say 100 yards) where the projectile path crosses the optical axis. It then reaches the highest point of it’s flight (when aiming perfectly horizontal) and then drops off, crossing the optical axis on the way down.
With most full power hunting rounds a 100m/yd zero roughly translates to a second zero at 200m/yd with a maximum offset of maybe 1,5″-2″ (~4cm) between these two which in turn gives you a drop off of 1,5″-2″ (4cm) at roughly 250m/yd – coupled with an over bore height of 1,5″-2″ that gives you a kill shot on a deer at up to 250m/yd without adjusting your aim.
Just an example.
(Fun fact: Out of a lever action .357 with a 20″ barrel most loads have a zero at 25m and 100m which means you can use the same sight setting on a pistol range and a rifle range 😀 )
Also the .30-30 has the ballistic of a brick. It may be a red-white-and-blue brick but it still is as aerodynamic as an equally all american Jeep.
With a .30-30 (not a Lever Evolution load) you might have a 75-120 zero but this round is rare in our neck of the woods so I don’t know for sure.
🙂 🙂 🙂 Now you’re cookin’ with fire. SFP scope subtensions are only accurate at max power, that’s why many prefer FFP – it’s just easier! As for the trajectory adjustments at different ranges, my take is mil dots work for ‘Kentuky drop’ calculations, but it’s less of a ‘calculation’ than it is an empirical ‘I know at 25 yards I need to shoot XX inches low because that’s where my rifle hits at that distance, and I’ve got the empty brass to prove it.’ good luck!
An argument for a fixed power scope, or a lower LPVO with the max mag where you want to be most of the time? Just some random thoughts.
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I believe there are ballistic calculators available that let you input the round, scope reticle parameters, etc, and give you a solution; but I’ve never used one so couldn’t make a recommendation.
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This is all a very good argument for either personal railguns or MW-class lasers. Either way, negligible drop under most circumstances one might encounter. (But the laser still suffers from crosswinds.)