The Big ACOG BDC Chart
by Bartholomew_Roberts
So you want a donut reticle; but you are planning on flattop mounting it to a 14.5" M4gery and are concerned the BDC will no longer be useful?
Never fear... someone with WAY too much time on their hands and a ballistics calculator has already figured out how to match your ammo/barrel/mount to your BDC.
Caveats:
I already caught two errors in my work. It is possible there is more. Feel free to check my work and correct it. Alternatively, if you rely on my work for some critical decision without checking it yourself first, you get what you deserve.
This isn't for anyone who wants High Power rifle standards of accuracy. You can't match the BDC exactly but these methods will put you within MOA of the BDC - close enough that you'll have to be one hell of a shooter to notice the difference.
Calculator only ran to 600 so ubershooters wanting a match out to 700 and 800 for your long-range M16 shooting are out of luck.
5.56mm Carry-Handle BDCs
All full size ACOGs for 5.56mm use this BDC with the exceptions noted below. It is designed around a 20" barrel firing M193 with the ACOG mounted in the carry handle. If you have one sight in according to your manual.
Stats used to figure BDC baseline:
M193 traveling at 3,259fps with a BC of .243 and a height over bore of 3.5"
For a Flattop 16" firing M193:
Sight in 1" high at 100m. You'll be 0.3" high at 200, 0.7" low at 300 and within MOA until 600 where you will be 10" low.
For a Flattop 14.5" firing M193:
Sight in per your manual but at 50yds (or sight in 1.5" high at 100m). You'll be high at 100; but virtually dead on until 600 where you'll be 12" low.
For a Carry Handle 20", 16" or 14.5" firing M855
Sight in per your manual at 100m. From a 20" you'll be within MOA of your BDC until 600 where you are 8" high. From a 16", you'll be almost dead on at all ranges. From a 14.5" you'll be within MOA.
For a Flattop 14.5" firing M855
Sight in at 60 (or sight in at 100 1.4" high). You'll be high at 100, 0.5" or less high at 200 and 300 and dead on the BDC out to 600.
For a Flattop 16" firing M855
Sight in 1" high at 100. You'll be a half inch high at 200 0.5" low at 300 and under 2" off BDC out to 600.
For a Flattop 20" firing M855
Sight in 0.8" high at 100 (or sight in at 60). You'll be 0.5" high at 200 and then dead on the BDC to 500 where you'll be 3" high and 600 you'll be 8.5" high.
EDITED TO ADD 75gr OTM data
For a Carry Handle 20" firing Hornady 75gr OTM @2750fps
Sight in 0.9" high at 100. You'll be 0.8" high at 200, 0.3" low at 300 and within 2" of BDC out to 600.
For a Flattop 20" firing Hornady 75gr OTM
Sight in at 50 (or 1.2" high at 100). You'll be 0.5" high at 200, 1.5" low at 300 and within MOA to 600
For a Carry Handle 16" firing 75gr OTM @2615fps
Two choices here:
Sight in 1.7" high at 100. You'll be dead on at 200 and 300, within MOA out to 600.
Sight in 1.2" high at 100, you'll be on the far end of MOA at all ranges out to 600.
For a Flattop 16" firing 75gr OTM
You'll have to sight in 2.2" high at 100. You'll be dead on at 200 and 300; and MOA out to 600. On the plus side, your close-in holdover will be a lot less.
No data for the 75gr out of a 14.5" barrel. I need a good muzzle velocity before I can figure that out; but the 16" should be close.
Fun facts:
Assuming you use M193, every inch you lower your scope mount will cause your round to strike 1" lower than the 55gr BDC at 200, 2" lower at 300, 3" at 400, 4" at 500, 5" at 600 etc.
62gr. 5.56mm Flattop BDC
ACOGS that use this BDC are the:
TA01NSN, TA31F, TA11F, TA11F-A
This BDC is based on a 14.5" barrel firing M855 with the ACOG mounted to the receiver via a TA51 mount. If you have one of these, sight in according to manual.
Stats used to figure BDC baseline:
M855 traveling at 2,970fps with a BC of .304 and a height over bore of 2.5"
For a Carry Handle 20" firing either M193 or M855 - you're screwed. There is no way you will match the BDC close enough to make it useful. You'll just have to learn where the rounds strike on your BDC and adjust.
For a Flattop 20" firing M193:
Sight in per your manual. You'll be within MOA of your BDC.
For a Carry Handle 16" or 14.5" firing M193:
Sight in .25" low at 100m. You'll be an 1" at 200, 2" high at 300 and 400 and then within MOA out to 600.
For a Flattop 16" firing M193:
Sight in at 100 per your manual. You'll be a half-inch high at 300, dead on at all other ranges until 500. At 500, the M193 starts losing juice fast on this trajectory, you'll be 2" low at 500 and 9" low by 600.
For a Flattop 14.5" firing M193:
Sight in at 100 per your manual. You'll be a half inch low at 300, 3" low at 400, 9" low at 500 and 15" low by 550. Once again, the 55gr loses gas fast past 300, so there isn't much you can do here without making your near zero absurdly high.
For a Flattop 16" firing M855:
Sight in per your manual. If you can notice this difference on the BDC you don't need to be asking me for advice.
For a Carry Handle 16" firing M855:
Sight in 0.1" high at 100 - this will give you a profile very close to the carry handle shooting M193.
EDITED TO ADD Hornady 75gr OTM Data
For a Carry Handle 20" firing 75gr OTM @2750fps
Sight in at 100 per manual. You'll be very close to the BDC with the maximum deviation being 4" high at 600
For a Flattop 20" firing 75gr OTM
Sight in at 100 per manual. You'll be on the low side of the BDC instead of the high side but within 2" or less all the way out to 600.
For a Carry Handle 16" firing 75gr OTM
Sight in 0.4" at 100 (or sight in normally at 75). You'll be a half inch high at 200, dead on at 300 and within MOA to 600.
For a Flattop 16" firing 75gr OTM
Sight in 1.1" high at 100. You'll be 1" high at 200, dead on at 300 and MOA until 600 where you will be 7" lower than the BDC.
I don't have muzzle velocity for the 14.5" so no figures for it.
Fun facts:
Using different mounts (#19S, TA51 on top of a rail, etc) changes the equation but not by much. So I didn't include that myriad of possibilities.
By the way Neil, a TA31 in a TA51 mount on top of a SIR and 14.5" is almost dead on the TA31 BDC at all other ranges if you sight an inch high at 100. (0.9" actually)
68gr and .308 reticles may or may not follow. I need muzzle velocity for the 68gr load out of 14.5", 16" and 20" barrels and I need to know the round used to calibrate the .308 ACOGs, we are guessing M852 match right now - if correct I still need a muzzle velocity for it out of a 20" barrel.