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Posted: 3/10/2005 6:37:37 PM EST
Hey does anyone have the ACOG data that B. Roberts had that was tacked on the optics forum??
Link Posted: 3/10/2005 7:49:01 PM EST
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.
Link Posted: 3/10/2005 8:23:41 PM EST
My TA31 came in day before yesterday. I thought that thread was gone for good! Robert 2011, I thank you from the bottom of my heart!
Link Posted: 3/10/2005 9:06:00 PM EST

It's no problem digging it up. Besides residing on my harddrive for safekeeping it is also available to gold team members on the "testarchive" server for now. I'm a team beta tester

testarchive.ar15.com/forums/topic.html?b=3&f=18&t=152794
Link Posted: 3/11/2005 5:25:55 AM EST
Do you happen to know what zero Trijicon assumes for 7.62 in the TA11? In other words, should I zero at 50 yards or 100 yards in order to come closest to matching the aiming points in the reticle at the various distances?

My very limited experimentation indicates that a 50 yard zero will put the bottom of the donut dead on at 300 yards with 168g Sierra BT match, so I am guessing Trijicon assumes a 50 yard zero.
Link Posted: 3/11/2005 1:41:47 PM EST
Link Posted: 3/11/2005 3:11:43 PM EST
According to the manual, top of the donut is the zero at 100, center at 200, then you are into the bdc. Problem is the donut bdc is designed for carry handle mount which means it is close but no cigar from the flat top. The chevron should work just fine from the flat top if you zero the tip of the chevron at 100. Good luck.
Link Posted: 3/11/2005 3:29:40 PM EST

Originally Posted By Robert2011:
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.



Isn't this BDC based on a 16" barrel?

Link Posted: 3/11/2005 3:41:28 PM EST
Link Posted: 3/11/2005 5:02:33 PM EST
Tag
Link Posted: 3/12/2005 6:39:47 AM EST

Originally Posted By sovereign:

Isn't this BDC based on a 16" barrel?



At the time of the original BDC thread, Trijicon had not published any information on the barrel length used for the flattop models. I assumed that because they were part of the SOPMOD kit, the barrel length was 14.5" and computed the BDC using the standard velocities for M855 from a 14.5" barrel.

Since that time, Trijicon has said the BDC is designed for a 16" barrel - at first I figured this was an error since they had erroneous info on their website previously; but it appears they did use velocities from a 16" rifle.

From a practical perspective, the difference at 600yds was a few inches, so I never bothered to change the original numbers as it was quite a bit of work to develop them in the first place at even at extreme ranges, the shift in point of impact was still smaller than the area obscured by the crosshair.
Link Posted: 3/12/2005 7:02:03 AM EST
[Last Edit: 3/12/2005 7:03:21 AM EST by Rmplstlskn]
I'm sorry if this has been posted and my search didn't find it, but...

Following the BDC chart above, using a 20" CH mount and M855, I should sight in at 100m per ACOG manual and it should be good or MOA all the way till 600m. All well and good...

Can any math geniuses tell me what my point of impact from point of aim would be at 100yds and at 50yds.? Mainly 50yds as this is convenient for the range I will be using. I don't know how to calculate BD with conversion of meters to yards. My point of aim will be the TIP of the TRIANGLE on a TA11D...

Thanks so much!!!

Rmpl
Link Posted: 3/13/2005 4:30:38 AM EST

Originally Posted By Rmplstlskn:
Can any math geniuses tell me what my point of impact from point of aim would be at 100yds and at 50yds.? Mainly 50yds as this is convenient for the range I will be using. I don't know how to calculate BD with conversion of meters to yards. My point of aim will be the TIP of the TRIANGLE on a TA11D



To sight in for 100m, use the tip of the triangle as your POA/POI at 100yds. On a 50yd range, use the center of the triangle (200m) aiming point as your POI. On a 25yd range use the bottom of the triangle (300m) as your POI.
Link Posted: 3/13/2005 5:25:52 AM EST
[Last Edit: 3/13/2005 5:27:34 AM EST by Rmplstlskn]
That was easy enough....

The yards -v- meters will still work out??? I'm assuming this is just a general conversion...

Rmpl
Link Posted: 3/13/2005 11:23:01 AM EST
At close ranges (100yds or less) the difference between yards and meters is small enough that it won't make any significant difference. If you sight in at 100yds instead of 100m, the difference in point of impact at 600m is 0.2"
Link Posted: 3/15/2005 10:30:44 AM EST

Originally Posted By AEM:
Do you happen to know what zero Trijicon assumes for 7.62 in the TA11? In other words, should I zero at 50 yards or 100 yards in order to come closest to matching the aiming points in the reticle at the various distances?

My very limited experimentation indicates that a 50 yard zero will put the bottom of the donut dead on at 300 yards with 168g Sierra BT match, so I am guessing Trijicon assumes a 50 yard zero.



What I am looking for here is whether I should zero dead on with .308 at 50 yds or 100 yds in order for the ranging points in the reticle to be most accurate.

Thanks
Link Posted: 3/15/2005 10:47:18 AM EST
I've got no experience with the .308 reticles. In the .223 ACOGs, the top of the reticle is sighted in at 100m/100y with the middle being the 50yd/200m point of aim and the bottom being the 25yd/300m point of aim. I would assume that the .308 ACOGs are set up the same way.
Link Posted: 3/15/2005 11:40:26 AM EST
That what it seems to be. A 50 yd. zero with 168g. seems to calibrate the donut , at least at 300 yds. Thanks
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