Sunday, July 8, 2012
.223 to .22 Ballistic Conversion
Here's a little bit of the premise so that if I'm wrong you can go ahead and correct me. I wanted a calculator that would allow me to swap out my ammunition and provide sizing data for targets in order to effectively train on shorter ranges with cheaper ammunition.
The first step is to be able to simulate a center of mass target on a range shorter than what you'd prefer to practice on. After I applied a little trigonometry, the calculator allows you to input the simulated range and the actual range and outputs the new target diameter. 19 inches is the diameter of a standard Army E-type silhouette to represent the center of mass. This is a generally accepted figure for training against human targets.
Point of Aim
Rifles are zeroed to hit a target at a certain range. In the case of the M-16 family, the Army zeroes to 300 meters. This means if you aim your iron sights on a rifle zeroed to your eyesight and a 300m target you'll hit where you aim. This also means that for any target closer or farther than 300m you won't hit where you aim. This isn't a problem, you just need to learn where to aim for certain distances and keep practicing.
At this point the bullet drop (or rise) is calculated for the full distance target. Using the same scaling trigonometry as on the target sizing, the point of aim is calculated for the simulated distance. It is important to note here, that on the simulated distance adjusting the point of aim will not cause the bullet to hit center of mass on the target like it would on the full distance target. That is why the next step is crucial, calculating the point of impact.
Point of Impact
To reinforce the point, the point of impact after properly adjusting the point of aim on a full distance target is the center of the target. On the simulated range, however, point of impact will be different depending on the trajectory. The calculator compensates for shooting both .223 and .22 on a shorter range. To tell if your shots would have hit their intended target, adjust a circle the size of the compensated diameter the specified distance from the center of your original target. (positive numbers go up, negative go down)
Page 5 of the spreadsheet has a crude version of the targets that will automatically redraw based on the inputs for the calculator. The targets are sized so the plot should scale onto a computer sized paper with no margins. The cross hairs represent the point of aim, the black "8" represents center of mass and head of the target, and the gray circle represents point of impact for the .22 rounds. I've also added a red ring that shows the size of a 65MOA circle at the range the targets are being used, note the ring will not be visible on targets beyond 18 meters when using 8.5x11" paper. The ring can be deleted from the spreadsheet if the shooter is not using an EOTech Holographic Weapon Sight.
The ballistic tables came from gundata.org. I used a .223 Remington, Remington Metal Case, 55gr for the benchmark and a .22 Long Rifle High Velocity and Hyper Velocity, Remington Long Rifle Solid, 40gr for the .22LR standard. The .223 was set at 300m zero and the .22 was set at 65yard zero as the projectile trajectories most closely matched in short distances from the barrel (up to 25 yards).
If you are set on a certain type of .22 ammo, check out gundata.org for the ballistic properties. Import the table into the spreadsheet in place of the ammo I have listed on page 2. This will change the polynomial function used to express bullet drop which you'll then need to manually change the equation in the respective cells on the calculator pages. It may sound hard, but I promise it's not. If I ever get a legit program made I'll be sure to account for more types of ammunition.
Again, if you have any experience writing programs or apps, contact us here. It truly is a tool that could be utilized by US soldiers if it is more extensively developed.
Feel free to let me know of any glaring problems with these formulas or my approach.
Page 1: .223 Raw Data
Page 2: .22LR Raw Data
Page 3: .223 vs .22 Ballistics at 100 yds
Page 4: Calculation Page: Meters
Page 5: Targets: Meters
Download the file here to use the spreadsheet. Note: spreadsheet will only work when downloaded as a MS Excel file, for some reason it doesn't perform correctly as a Google spreadsheet.
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