HELP! Bullet Drop Calculations Not Consistent with Actual POI

[RCBorries]

Hello, I am new to forum and although I have been a avid shooter most of 33 years of life, I am not by no means experienced in shooting long distances over 400 yards. After deciding to take a break from hunting this this season to concentrate our newest addition to the family and first child, I have been doing more shooting as a hobby.

Anyways, I am having an issue with consistency of elevation calculations vs actual POI so I am hoping you guys can help me out with this. To be more specific, my issues start out at 300 and 400 yards and my point of impacts are lower than what the BC calculations say they should be at these distances. I am sure this inconsistency would grow with distance but 400 yard is the longest shot I have on my property where I am shooting.

The rifle is, for the most part, an all original Colt AR-15 16” barrel with a 1:7 twist. Ammo is Hornady Black 62 gr 223. The optic is an ATN X-Sight 4k Pro 3-14 (X-mas present for myself!). For those who are not familiar with the scope, it is a digital Day /Night Scope that has a built in ballistic calculator.

I have made a profile for this weapon and ammo in the scope and double checked to make sure all the info was entered correct (Velocity, BC, Sight height, Zero distance, etc.). I have it is zeroed at 100yds. At 200 yards my POI and groups are as they should be but There is not much drop in the bullet to matter there anyways. With the ballistic calculator set for 300 yards the POI is consistently 5-7 inches lower than what the BC calculator is adjusting for. At 400 yards the POI is 12-14 inches lower than what the BC calculator is adjusting for. Thinking this could be an error in the scopes BC calculator, I plugged the numbers into the Hornady BC Calculator App and I am getting the same elevation adjustments as the scope is giving me. I have checked the scope for level and even re-mounted it on the rifle to ensure it was level (using a scope level) doubled checked the sight height, re-zeroed the scope, double and triple checked all the calculation input data as well as made sure the scope was not set on meters or other units of measurement. I have also checked angle of my shot, which according to the scopes built in angle calculator is 0 and by looking at the target downrange it seems to be accurate “enough” that it would not cause that much of a variation in bullet drop as I am experiencing. Any help on this would be greatly appreciated.

BTW Please excuse my terminology/lingo if it is not accurate, as I am somewhat new to this and I am by no means experienced in the long range shooting field. Thanks.

 

[Rocketvapor]

What velocity, Published or actually measured?
3000fps actual and entering the G1 B.C. as a G7 B.C. would give you about that much error.

 

[Steel head]

If it’s a consistent occurrence then true your BC calc using BC or speed or a combination of both.

 

[RCBorries]

What velocity, Published or actually measured?
3000fps actual and entering the G1 B.C. as a G7 B.C. would give you about that much error.

3100 is what is published. on the Hornady BC app and when you click on the little question mark beside the muzzle velocity in the app it states "Actual muzzle velocity corrected to the muzzle from the chronograph location". I assume this is answered your questions?

Just check the profile i made on Hornady BC App to compare to my scope calculations and i am using the G1 BC. I will check this again in the scope BC calculator well when i get home but I am 90% sure it was set on G1 as well. On Second thought, being they are both giving me the same calculations I am 99% sure the scope is set to G1 as well.

 

[RCBorries]

If it’s a consistent occurrence then true your BC calc using BC or speed or a combination of both.

I could most definitely manipulate it to make it work but that wouldnt do much for me as far as knowing why the cal is not giving me accurate adjustments. And being I am still new to this and learning, that method would not give me much faith in being able to accurately calculate shots in the future. Whether its something im doing wrong or if its the rifle or its the ammo, etc I would just like to know. I mean, maybe I am too OCD? Is this method you suggest something that is typically done when shooting long range? Are these inconsistencies common?

 

[Steel head]

Advertised BC is just that.
It’s not gospel.
Have you verified bullet speed?

 

[HiDesertELR]

Keep track of your DOPE and "Trust the bullet"

Post a screen shot of the ballistic app too, might be a clue in there for someone to better help you.

Also, check this out:

Marc's Weaponized Math

Here is the link to the article on the Math we discussed in the Podcast from Alaska https://www.snipershide.com/predictable-weaponized-math-and-x-factor-voodoo/ Weaponized Math Article Take my 6.5, for instance, at my home range: 8.7 at 1,000 minus 5.8 at 800 equals 2.9 Mils of drop in that...

www.snipershide.com

 

[Sheldon N]

Adjust your velocity to bring the 300 and 400 yard data into alignment with actual measured impacts, then fine tune anything at 500 and 600 yards with BC adjustments if needed.

BC will dramatically affect the ballistic calculator data at farther distances, it has less of an effect on closer range.

 

[RCBorries]

Advertised BC is just that.
It’s not gospel.
Have you verified bullet speed?

Ok, so basically some provided variables like published muzzle velocity could be off. I guess I was putting too much faith into the information provided on the box of ammo. To be honest I was not even aware there was a substantial difference that needed to be considered between actual and published velocities until you guys enlightened me just now. Learning new things already! So i took your advise and played with the velocity and if i go down to 2675fps (from 3100) both the 300yrd and 400yrd are right in line of actual adjustments needed.

No I have not verified bullet speed. I assume you are referring to running them in a choreograph, which i do not have.

Thanks for the help and advise!

 

[RCBorries]

Keep track of your DOPE and "Trust the bullet"

Post a screen shot of the ballistic app too, might be a clue in there for someone to better help you.

Also, check this out:

Marc's Weaponized Math

Here is the link to the article on the Math we discussed in the Podcast from Alaska https://www.snipershide.com/predictable-weaponized-math-and-x-factor-voodoo/ Weaponized Math Article Take my 6.5, for instance, at my home range: 8.7 at 1,000 minus 5.8 at 800 equals 2.9 Mils of drop in that...

www.snipershide.com

Thanks for the help and the link. I will definitely read up on this!

 

[RCBorries]

Adjust your velocity to bring the 300 and 400 yard data into alignment with actual measured impacts, then fine tune anything at 500 and 600 yards with BC adjustments if needed.

BC will dramatically affect the ballistic calculator data at farther distances, it has less of an effect on closer range.

This is a very helpful piece of info I will have to remember and play with when making adjustments. Thank you

 

[Steel head]

Ok, so basically some provided variables like published muzzle velocity could be off. I guess I was putting too much faith into the information provided on the box of ammo. To be honest I was not even aware there was a substantial difference that needed to be considered between actual and published velocities until you guys enlightened me just now. Learning new things already! So i took your advise and played with the velocity and if i go down to 2675fps (from 3100) both the 300yrd and 400yrd are right in line of actual adjustments needed.

No I have not verified bullet speed. I assume you are referring to running them in a choreograph, which i do not have.

Thanks for the help and advise!

Yes.
box information is just a general guideline.

Sheldon was on it as well about using speeds at shorter ranges and BC at further ranges.

 

[RCBorries]

So it seems my biggest factor of error that you guys have helped with identifying so far was using the published velocity to calculate shots. I also have a hunch that not having the ability to accurately account for barrel length in my cals was a contributor as well. Which in my case directly relates back to loss of velocuty being Hornady based their results on using a 24" barrel for this particular ammo and my barrel being 16" Correct?

That said, being my elevation cals were off by 13" at only 400 yards, using these general sources of info to generate POI calculation at much furhter distances is basically useless. For example, if i were using the same general inputs at say 3x the distance (1200 yards) I could very well be off by 3x that much or more when considering the increase in energy loss at longer distances. Your talking 39 inches at minimum that your cals would be off! Seems like you would never get on target with that much error! Am I on the right track here? If so, that means knowing the actual velocity is a absolute MUST for shots at those distances. And thats just one factor to consider!!!! MAN my head is hurting! Very interesting though...

 

[Rocketvapor]

Knowing actual velocity would be best.
A 16" guess might help some but not 13" @400 yards.
More like 8 to 9".
Unless you have a really slow barrel.

 

[stello1001]

You need to put in the work and time and actually dope your rifle out. Even after I true my MV by using bullet drop at a certain distance, my ballistic app still spits out drops at other distances that are not true.

Dope your rifle out at varying distance and take note of your actual drops. That's as true as you can get...

 

[Sheldon N]

That said, being my elevation cals were off by 13" at only 400 yards, using these general sources of info to generate POI calculation at much furhter distances is basically useless. For example, if i were using the same general inputs at say 3x the distance (1200 yards) I could very well be off by 3x that much or more when considering the increase in energy loss at longer distances. Your talking 39 inches at minimum that your cals would be off! Seems like you would never get on target with that much error! Am I on the right track here? If so, that means knowing the actual velocity is a absolute MUST for shots at those distances. And thats just one factor to consider!!!! MAN my head is hurting! Very interesting though...

Once you get the basic inputs correct (scope height over bore, solid zero, properly functioning scope tracking, correct environmentals - altitude, temp, pressure, humidity) then the goal is to use your measured impacts at confirmed distances to backwards calculate your velocity and your bullet's BC. Once you have those two things calculated properly, then you can take the gun anywhere and get impacts at most any distance.

I like to use an online calculator to assist with this process. It's quick and easy to change things and see what the effect is. The JBM calculator is a great resource for this.

JBM - Calculations - Trajectory (Simplified)

www.jbmballistics.com

The thing to keep in mind is that both velocity and BC affect the calculation and they interplay. However, adjusting BC doesn't effect the close distances very much so the general rule of thumb is to adjust velocity for the close shots then adjust BC to align the longer range shots. Roughly speaking close range would be anything where the bullet is going faster than 1500 fps, and the longer range BC adjustment would be done where the bullet speed has dropped down getting closer to transonic/subsonic.

Get the velocity right and the BC right so the calculator matches your measured impacts at distance - that's "truing up" your data. Then with that you can adjust environmental conditions in your ballistic app and still get impacts regardless of the air density where you are shooting.

 

[RCBorries]

That will be on my for this weekend then. Dope my rifle out and true up my data. Playing around with this AR just to get my feet wet in the hobby until I make my decision on which 6.5cm platform I want to buy. Purchased a brand-R-new Nikon Black fx1000 a few weeks back to go on it that im pretty excited about!