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GettoPhilosopher
04-20-2012, 8:59 AM
Ok guys, help me out here. I don't know if I'm missing something, if I screwed up my calculations, or what, but I can't for the life of me get the drops right for an ACOG. (I'm using JBM Ballistics)

According to the ACOG Chart (http://www.ar15.com/forums/t_3_18/229420_Bartholomew_Roberts__Big_ACOG_BDC_Chart.htm l), the TA11F is calibrated for a 62gr M855 round out of a 14.5" barrel with a sight height of 2.5".

However, later posts show that has changed, and it's now calibrated for a 20" barrel.

I called Trijicon to confirm. They gave me both the calibration data (3050fps, 62gr bullet, 20" barrel) AND the MOA drops of the reticle. They are as follows:

100: 0
200: -2 MOA
300: -4.4 MOA
400: -7.8 MOA
500: -12.6 MOA
600: -18.6 MOA
700: -26.4 MOA
800: -35.8 MOA

Problem is, when I plug in that data, I get totally different drop values.

100:-0.0 MOA
200:-1.4 MOA
300:-4.0 MOA
400:-7.4 MOA
500:-11.6 MOA
600:-16.9 MOA
700:-23.5 MOA
800:-31.7 MOA

On snipershide, someone posted even WEIRDER values:

100: 0 MOA
200: 1.5 MOA
300: 3.5 MOA
400: 6.6 MOA
500: 10.5 MOA
600: 15.1 MOA
700: 20.9 MOA
800: 28.0 MOA

I've tried it with my Android "Shooter" app as well, I get similar results to JBM. I've also tried varying the values (3100FPS, 3000FPS, 2970 FPS....2" sight height, 2.75", 3"....etc). I can't for the life of me make the drop values provided by Trijicon make any sense.

Help? Even if I just shrug and accept their drop values as accurate (which I do), if I don't know what I'm doing wrong I can't trust the values I get for my rifle/ammo.

SuperSet
04-20-2012, 9:13 AM
Now that you know the subtensions for your ACOG reticle, shouldn't the next step be to see what your current rifle + load is performing and seeing how that matches to the reticle? For example, if you plug in your FPS, height over bore, and bullet choice, what range does -2 and -4.4 MOA equate to?

GettoPhilosopher
04-20-2012, 9:17 AM
Now that you know the subtensions for your ACOG reticle, shouldn't the next step be to see what your current rifle + load is performing and seeing how that matches to the reticle? For example, if you plug in your FPS, height over bore, and bullet choice, what range does -2 and -4.4 MOA equate to?

Yes, but the problem is when I try to calculate it out, Trijicon's subtensions don't make any danged sense compared to what they say it's calibrated for.

I have two options; assume Trijicon is incompetent, or assume I'm screwing up. I have to assume I'm screwing up.

So if I'm screwing up and can't figure out what I'm doing wrong, I won't be able to trust my results for my current rifle + load to match to the reticle, because I'll probably be making the same mistake(s?) when calculating my rifle and my load.

chicoredneck
04-20-2012, 9:31 AM
Your not "screwing up" and trijicon is not incompetent. You must shoot your load at the ranges you intend to shoot it at and measure your own drops. The numbers trijicon provided are just a guide. As it pertains to individual rifles and atmospheric conditions trijicons numbers are really just a WAG. JBM ballistics will get you numbers close to your rifles ballistics, but not exact. You have to take it out and shoot it with the ammo you use, measure and record your drops at various ranges, and then apply those numbers to your scope subtentions.

The data you gather will only be exact in similar atmospheric conditions to those that you collected your data in. A change of just 2000 ft in elevation and swings of 30*F can change your POI significantly past 500/600 yards. If the powder in your ammo is not tempreature insensitive it further componds the change you will see over varying conditions.

chicoredneck
04-20-2012, 9:35 AM
Once you have established your OWN rifles actual MOA drops, you need to adjust your ballistics program to match your real life data.

GettoPhilosopher
04-20-2012, 9:40 AM
So the consensus so far is that Trijicon's values are just up to 4 MOA off? That's 35 inches off at 800m. I find it hard to believe they'd be that inaccurate....

GettoPhilosopher
04-20-2012, 9:45 AM
I'm not sure if I'm explaining my concern correctly, so let me try again.

For a BDC reticle, short of building a dope book for every load, every rifle, and every condition, the steps go like this:

1) Figure out what the reticle is calibrated for. (__gr ___ caliber with ___ BC @ ___MV and ___sight height).
2) Calculate out the MOA drops of the reticle. Now you know what the reticle is actually measuring.
3) Plug in your rifle's ballistics into a reliable calculator.
4) See how your rifle/load's ballistics in your environment lines up with the BDC. So if my target is 400y out, and my drop at 400y is 5 MOA (I'm just making up numbers here), and the drop values for 300m and 400m are 4 MOA and 6 MOA respectively, I know to aim halfway between the 300m and 400m hashmarks to hit my target.

Now seriously, I get it. The ACOG is a minute-of-man scope. I'm not expecting to dial it in like my Viper PST and nail a 3" group at 400 yards. But if my reticle values are that far off, I will miss even a man-sized target anywhere further than 200-300yards, and that's not acceptable to me.

So why would my calculations for their stated calibration (62gr M855 at 3050 FPS with a 2.5" sight height and a .305 BC) be 4 MOA off their stated drop values?

DirtRacer151
04-20-2012, 10:03 AM
What 62gr bullet are they using?

BDC reticles are a "rough" estimate. Climate, load, and the.rifle itself will all play a role.

What you need to do is get a range finder and a kestryl (auto atmosphere on shooter is NOT good enough) . Go work up an nice accurate load at 100yds. Then go record your rifles actual drop at 800yds. Then figure out where its hitting on the 800yd hash. It might be 750 or 850. Who knows... But once you figure this out you can establish new values for the BDC reticle. If you find that your load is impacting on the 800yd hash at 780 in reality, use shooter and input the correct atmosphere and 780yds. Then produce a table. Hit menu and use the muzzle velocity tool to adjust it up and down until your 780yd dope matches with the subtension of the 800yd hash in MOA (35.8 moa according to trijicon). Once you've got the muzzle velocity to match everything up then double check where all the other subtensions line up at and test them out. So 12.5 @ 500 subtension might actually be 490? on your setup. Test it to make sure Everything else lines up.

This is why you don't see long range shooters using BDC reticles. Too many changing conditions to have a reticle that's always calibrated. Once I know the atmosphere and the range ill input it to shooter and dial the knobs or hold over on a mil based reticle.

I'll tape dope to my rifle for a round about atmosphere that's somewhere in the middle of what I expect to be shooting in. In a hurry I can check that and use a holdover to make a semi accurate hit at beyond 500yds. If you want it to be dead nuts though you really need accurate atmosphere at the time of the shot though. 223 is even worse since it gets badly blown around and effected by density altitude even sooner then the higher BC 6.5, 6, or 308 bullets.

You're asking a lot of your system. I couldn't even get 800yd line drawn on my reticle for my 260 and expect it to always be dead on at 800yds. One day it would be dead on.. the next I'd be missing like no other. Your 223 will be effected even worse.

GettoPhilosopher
04-20-2012, 10:19 AM
What 62gr bullet are they using?

BDC reticles are a "rough" estimate. Climate, load, and the.rifle itself will all play a role.

They say a standard mil issue M855 round. I assumed they'd calibrate at "standard atmosphere" at sea level, but maybe that's my mistake?


What you need to do is get a range finder and a kestryl (auto atmosphere on shooter is NOT good enough) . Go work up an nice accurate load at 100yds. Then go record your rifles actual drop at 800yds. Then figure out where its hitting on the 800yd hash. It might be 750 or 850. Who knows... But once you figure this out you can establish new values for the BDC reticle. If you find that your load is impacting on the 800yd hash at 780 in reality, use shooter and input the correct atmosphere and 780yds. Then produce a table. Hit menu and use the muzzle velocity tool to adjust it up and down until your 780yd dope matches with the subtension of the 800yd hash in MOA (35.8 moa according to trijicon). Once you've got the muzzle velocity to match everything up then double check where all the other subtensions line up at and test them out. So 12.5 @ 500 subtension might actually be 490? on your setup. Test it to make sure Everything else lines up.

This is why you don't see long range shooters using BDC reticles. Too many changing conditions to have a reticle that's always calibrated. Once I know the atmosphere and the range ill input it to shooter and dial the knobs or hold over on a mil based reticle.

I'll tape dope to my rifle for a round about atmosphere that's somewhere in the middle of what I expect to be shooting in. In a hurry I can check that and use a holdover to make a semi accurate hit at beyond 500yds. If you want it to be dead nuts though you really need accurate atmosphere at the time of the shot though. 223 is even worse since it gets badly blown around and effected by density altitude even sooner then the higher BC 6.5, 6, or 308 bullets.

Thanks for the advice. I agree that it's not really designed for long range shooting, and that's not what I have the rifle for. It's currently on a SDM-R clone as a "spotter's rifle" for my wife. I also have an AR308 precision build with a Viper PST 4-16 FFP MIL, which we'll actually be using for learning precision shooting, doing holdovers, shooting long range, etc. I'm planning on taking some courses/clinics, ammo money permitting. The idea of the SDM-R is more steel targets at Angeles out to 600y...very much "minute of 30 inch plate".

Half the reason I care is because if I'm screwing something up fundamentally in my use of ballistics calculators, then I wouldn't be able to trust my soft dope on any rifle, even if I *did* have a kestryl and LRF (they're planned purchases, by the way), y'know?

DirtRacer151
04-20-2012, 10:46 AM
Register for our forum. We all go shooting quite frequently. I can give you a crash course and get you on with both rifles in no time.