How do I calculate the correct MOA drop?

T

tpcollins

New member
I have a firearm with a Swarovski 3-9x36mm with the 4A reticle. The subtension at 9 power from the crosshair center down to the edge of the heavy line on the vertical is listed at 8.3 MOA. My POI at 100 yards is 1.5 MOA high.

If I want to use the edge of the heavy vertical (8.3 MOA) as a "BDC" holdover, do I subtract the 1.5 from 8.3 (6.8 MOA) and use that yardage distance or do I just use the distance indicated for the entire 8.3 MOA? Thanks.
 
Go to the Berger Bullet Website and download their FREE ballistic program.

http://www.bergerbullets.com/Ballistics_Program/index.html

Input the data to the program and you'll get your answer. Like anything else, the more accurate your imputed data, the more accurate the results.

Find the BC of the bullet you're using, get the velocity. Measure the height of your sight above the bore (this is more important then people think).


If you don't have a crony, estimate the velocity using a reloading manual, use the estimate for the BC program, THEN go out and shoot it at different ranges. Using your shot groups at different ranges, (high or low compared to the point of aim) then playing with the programs velocity input until you get the same drop/rise will get you pretty close velocity wise.

Check the weather on your computer to get the environmental data needed it.

Remember, garbage in, garbage out. The more accurate the input data the more accurate the output.

I also use the Shooter Program on my Ipod. If I put in the same data as I put into Berger's program, I get the same out put from the two programs.

Forgot to add: ALWAYS check your data on the range, BC programs will get you close, actual shooting will get you closer.
 
What caliber is the rifle? Most high powered rifles and calibers these days sight 1.5 high at 100 are going to be very close at 200 and 6 to ten inches low at three hundred.

I would say use the same cross hair out to your zero range and then the full moa measurement past that. However what ever your moa measurement is will increase at longer range. Example if it is 8 moa that is about 8 inches at 100 yards the same 8 moa becomes 24 inches at 300 yards. It seems that with an 8 moa difference would not really be usefull until about 500 yards. It would be too much holdover much before then.
 
I do not quite understand the question.

Sighted in 1.5 moa above the crosshair, so 9.8 moa above the top of the post. So, at 100 yards, poi is 10.3" above the top of the post. You'd have to shoot it to verify.

Plug the numbers into a ballistics calculator and see where that puts you.
 
I've plugged in the numbers already, I know the drop- the type of rifle I'm using is irrelevant. If my subtension of the reticle is 8.3 MOA at 100 yards, and the POI is 1.5 MOA high on the bull at 100 yards using the center of the crosshair, to find the yardage on my ballistics chart for using the heavy section of the vertical line as a BDC, do I subtract 1.5 from 8.3 or not? The answer is either yes or no - thanks.
 
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If I understand you correclty . You are trying to find the distance to a known sized target using your reticle like a mil-dot reticle ?

If you are using the 8.3 MOA measurement to find the distance to a known sized target you will use the full 8.3 MOA . 8'3MOA is about 2.5 mils
 
Yes. If I understand you correctly.

But if you're wanting to make a trajectory chart using the top of the post as an aiming point, just forget all about the 1.5, and the 8.3, and the crosshair.

Determine the range at which you are zeroed using the top of the post, and figure your trajectory chart from that.
 
No you have to add 1.5" to your moa. at 100 yards your bullet impact is 8.3 moa +1.5 from the change in your reticle. To use 8.3 moa in the ballistic calculator sight in your poi dead on center crosshair. Or the calculation will be off. If the calculator has a place to enter poi above or below use 8.3. If not the 1.5" will throw the calculation off.
 
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SSA said: I do not quite understand the question.

Sighted in 1.5 moa above the crosshair, so 9.8 moa above the top of the post. So, at 100 yards, poi is 10.3" above the top of the post. You'd have to shoot it to verify.

Plug the numbers into a ballistics calculator and see where that puts you

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That make the most sense. I didn't want to mention that this was with a muzzleloader - I was hoping to just ask a MOA question without getting my ass shot off.

I want to hit 1.5 MOA high at 100 yards to increase my effective range and minimize holdover. I agree if I aimed at the bullseye with the top edge of the post, I should hit 9.8 MOA high at 100 yards.

I've entered all of my data into a ballistics program and set it at 1 yard increments. With a 1.5 MOA high at 100 yards, the zero range is 127 yards. If I want to find out at what yardage the bullet hits the bullseye using the top edge of the post, do I just use 8.3 MOA on the chart or do I add or subtract 1.5 from 8.3? Thanks.
 
That 1.5 above the line of sight convolutes the calculation because the trajectory changes significantly. Start with 8.3 and shoot it to confirm. It will be off, but then you will be able to calculate the difference and shoot again to verify.
 
Tell the calculator that you're sighted in at 225 yards, see where it says that puts you at 100, and adjust the input zero range accordingly.
 
With your load what range does the calculator show for the post at 8.3 moa? Just guessing at this but based on my muzzleloader experience it is around 250ish yards?
 
@100 yards you are 1.5 MOA high. That adds to the 8.3 between the crosshair and the heavy line. Go to your ballistic calculator and find the distance that gives you 9.8 MOA drop from a 100 yard zero. I bet it will be further away than you have any business hunting at.
 
I re-read your last post. I missed an important detail. Simply put yes you can calculate gr8.3 moa accurately with this data. Use 127 yards as your zero range, not 100. That should get you close.
 
Thanks SSA & Big Al - the 9.8 MOA above the bull (10.26" if we're getting technical) using the post makes sense to me now. I want my POI at 100 yards to be 1.5 MOA high, this gives me a zero yardage of 127 yards. An 8.3 MOA drop on my ballistics chart indicates an estimated POI on the bull at 189 yards.

I'm all set, thanks.
 
tpcollins, sorry I didn't see that you were shooting muzzle-loader, so that 9.8 MOA is a lot closer than the 500+ yards I was thinking for centerfire.

Still, be careful with your range estimation and hold-over onto a deer. It wouldn't take a lot of miscalculation to miss the vitals.
 
"15 inches ?? I get about 8.7 inches."

Rough numbers only, 8.5 MOA at 200 yards is 17 inches (because MOA is an angle, it gets larger the further away you are looking.) So 15 inches out at 189y seems reasonable.

It's only coincidence that 1 MOA is really close to 1 inch at 100 yards.

A good exercise might be for you to calculate what the body of a deer might look like compared with your reticle at various distances.
 
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