View Full Version : Height over bore and point of impact
Is there a formula one can use to determine the POA and POI spread based on the height of the optic over bore for different yardage/meters.
For example, Arma-Dynamics has a nice red dot zeroing target where you just need lace it at 25 yrds and aim for the red dot while adjusting for the impact on the grey dot below. So you can get a 50yrd zero at 25 yrds. http://www.arma-dynamics.com/assets/red-dot-25yd-mod1_1-(ar-15).pdf
These targets are set for 2.6-2.8” HOB.
Running a 1.93 height optic produces 3.22” HOB. So at 25 yrds, What would the spread be between POA & POI??? What’s the formula? Google wasn’t any help.
I have no idea -- there are so many variations on a theme out there (to include different base heights for standard sighting, vice 1.93" to see over a laser-illuminator on your top rail).
Get whatever sight base you want that meets what you need it to do. Shoot and center a group at 25, 36, or 50 yards to get it close to what you think your off-set is.
Then pick where you think the majority of your most likely and most dangerous targets will ever be. Is it dangerous game? Is it armored? Will it shoot back and kill your ass through your cover, concealment, and armor?
Is that range 25 yards? 100? 200? 300? THAT'S where you need to shoot, center, and confirm your groups for boring day-in / day-out / until the sun stops shining shooting.
Is there a formula one can use to determine the POA and POI spread based on the height of the optic over bore for different yardage/meters.
For example, Arma-Dynamics has a nice red dot zeroing target where you just need lace it at 25 yrds and aim for the red dot while adjusting for the impact on the grey dot below. So you can get a 50yrd zero at 25 yrds. http://www.arma-dynamics.com/assets/red-dot-25yd-mod1_1-(ar-15).pdf
These targets are set for 2.6-2.8” HOB.
Running a 1.93 height optic produces 3.22” HOB. So at 25 yrds, What would the spread be between POA & POI??? What’s the formula? Google wasn’t any help.
Your bullet will hit about 1.4" below point of aim at 25 yds. So aim for the middle of a forehead and you'll drop the round between the eyes of someone who deserves it.
My sight height is 2.6 HOB and I hit 1.1" below POA @ 25 yds.
This is with my 300 Blackout and 2350 FPS supers. But it won't matter much at these close ranges even if your using fast 5.56mm rounds.
Just use JBM Simplified Trajectory and plug in your data.
http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi
There are a ton of phone apps that will do the math for you.
This site should do what you are asking; select "Advanced options" http://gundata.org/ballistic-calculator/
Andy
POI should be -1" at 25 yards for a 50 yard zero with the sight plane 2.6" over the center of the bore axis. 3.22"/2.6" = 1.2385. Sighting your carbine to hit 1-1/4" low at 25 yards would be a good starting point.
Don't count on height over bore axis posts on the internet. Physically measure the distance from center of bore to center of optic or iron sight before plugging in numbers on a ballistic calculator.
https://shooterscalculator.com/ballistic-trajectory-chart.php
bamashooter
01-30-22, 07:24
In a controlled environment you can get really decent "modeled" returns. Just depends on the precision you're hoping for / expecting along with the consistency you seek. Additionally, some advanced calculators allow for the whole nine yards like custom loads, etc, whereas some just run basic-data algorithms such as height, weight, distance, etc.
There is good software out there to give numbers. But the "BULLET DON'T LIE", so just shoot it if you can and you will know 100%. It's not and exhausting project. Best of luck. And the bullet / round you chose will differ.
PB
Running a 1.93 height optic produces 3.22” HOB.
How’d you obtain that?
Went through this recently zeroing a T1/LPVO combo for a PSCL match:
1) Found minimum target size - 6x6 sq steel, A/K zone for paper was about same, max range was "sub 200", so guessed there would be a 200+ challenge shot.
2) Used https://shooterscalculator.com/ (same as T2C - Found it from an OLD recommendation here) & http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi - both were close with same input #'s
3) used public data and ballistics by the inch to find guestamate velocity for the rifle which I will never use in dusty conditions again
3) 25 -100 -300 yrds walk out to confirm zero and scope mounting not shifting (~50 rounds) and then back to 25yrd bay to shoot 3 yrds out to 25 yrds in increments for both 100 yrd zero on T1 and ~200 what ever gave me the best MPBR on the LPVO.
Don't count on height over bore axis posts on the internet. Physically measure the distance from center of bore to center of optic or iron sight before plugging in numbers on a ballistic calculator.
It might be helpful to know that the top surface of the rail on the standard mil-spec flat top AR upper is 1.250 inches above the center line of the bore. With that information, a lot of people will find it much easier to measure the actual site height center over bore center for a particular optic.
When you plug numbers into a ballistic calculator, small changes in the site height over bore don't make a big difference. But it's probably a good idea to be as exact as possible to get the most helpful information out of the ballistic calculators.
Joe Mamma
There is good software out there to give numbers. But the "BULLET DON'T LIE", so just shoot it if you can and you will know 100%. It snot and exhausting project. Best of luck. And the bullet / round you chose will differ.
PB
This. Nothing is going to beat real world data collection.
This. Nothing is going to beat real world data collection.
That's a fact. Several years ago I ran some numbers on RemShoot software to determine POI for an iron sight AR-15 carbine in 25 yard increments using M193 ball ammunition. I physically shot from 25 yards to 300 yards in 25 yard increments and the POI was within 1" of what the RemShoot software calculated it would be.
A free billistic app on your phone will get you very close, but as said - verify on the range at a few distances.
A chrono will help accuracy as well.
8-1/2 years of new stuff, but still good info.
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