This is no longer available except on the archieve server which is only available to Team Members, so I thought I would repost it and tack it for everyone.
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Originally posted by Bartholomew_Roberts 03/03. A big thanks is owed to him for taking the time to put together this information


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.


INSTRUCTIONS FOR SIGHTING IN YOUR RIFLE

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.

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, there are two choices:
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.

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.308 Carry-Handle BDC

• This BDC is based on a carry handle ACOG firing M852 Match out of a 20" barrel.
• ACOGs using this BDC include the TA11C and TA01B.
• Stats used to figure BDC baseline: M852 traveling at 2,500fps with a BC of .447 and a height over bore of 3.5"

For a Flattop 20" barrel firing M852:
Sight in at 150 or 0.7" high at 100. You will not deviate more than 0.5" from the BDC all the way out to 600

For a Flattop 20" firing 75gr OTM @2750fps
Sight in at 100 per manual. You'll be dead on to 200, 1" high at 300, 3" high at 400, 6.7" at 500, and a bit over 10" high at 600.

For a Carry Handle 20" firing 75gr OTM
Sight in at 100 per manual. This doesn't match the .308 BDC too well; but it is the best you'll get. 1" high at 200, 3" high at 300, 5" high at 400, 10" at 500 and 15" high at 600.

For a Flattop 16" firing 75gr OTM @2615fps
Sight in at 100 per manual. This is a real close match. Max deviation is 2" at 600m and you'll be within 1" at all other ranges.

For a Carry Handle 16" firing 75gr OTM
Sight in at 100 per manual (for a slight edge, sight in at 125 but if you are good enough to notice the edge, then you probably want a more precise match anyway. You'll be about half MOA out to 600.

• There is no data for other .308 rounds or 68gr OTM because I don't have the necessary info to figure it out. If somebody gets me the 68gr info I'll add it; but those wishing SMK data or additional .308 data will need to run the numbers themselves (although I can offer help if you need it).
• There is no data for 55gr or 62gr because the .308 BDC is by and large a bad match for these rounds.
• As you can see the .308 BDC is an excellent match for the 5.56mm 75gr OTM in most applications.
• The TA11E uses a flattop adapter. If the BDC is designed for flattop mounting (2.5" height over bore), this ACOG will be more difficult to match to 75gr OTM use. Use the sight-in procedures given for the regular .308 BDC but figure an extra 1" high at 200, 2" high at 300, 3" high at 400 and so on.

Originally Posted By sovereign: From a phone conversation with Trijicon to confirm the BDC specs, I was told the following data were used to develop the BDC: Flat Top Models: 14" barrel, 62gr., 3050 FPS Carry Handle Models: 20" barrel, 55gr., 3120 FPS

Hmmmm, odd that Trijicon is managing to get about an extra 80fps out of their 14.5" barrels according to the info they gave you. Looks more like they really did calibrate it using a 16" or even 20" barrel.

When the chart was put together, we had no input from Trijicon beyond the fact that the F-series were designed for 62gr M855 from a 14.5" flattop and the rest were for 55gr M193 from a 20" carry handle. I used the normal average velocities for these rounds from those barrels to compute the BDC chart.

Anybody who would like to redo the whole chart using the new values is most welcome to do so, but I think most people will find the differences to be pretty minimal.

Looks like they used SAAMI 55gr .223 since I show better velocities than that for a 20" firing M193.

I notice several questions regarding zeroing the ACOG at shorter distances.

Here are the aim points at 100, 200 and 300m on the ACOG. On the donut, this is top of donut, middle of donut, bottom of donut. On the chevron, this is top of chevron, bottom of chevron point, and small illuminated portion of ladder below the chevron. Triangle is tip of triangle, middle of triangle, bottom of triangle (see a pattern here?).

Actual zero range is 25yds = zero as above using 300m reference as point of aim
Actual zero range is 50yds = zero as above using 200m reference as point of aim

If you are attempting to zero using the charts above, you will also need to convert to MOA and adjust your point of impact accordingly. Example:

For a Carry Handle 16" or 14.5" firing M193 but zeroing at 50yds:

Normally, you would sight in .25" low at 100m and be 1" high at 200, 2" high at 300 and 400 and then within MOA out to 600. Since you are sighting inat 50yds, you take the difference at 200 (0.5MOA) and apply it at 50yds (1/4" high above your 200m point of aim).

As always, these are rough estimates designed to get you on target quickly. You should always verify the zero at the appropriate ranges in the environmental conditions you plan to use the rifle in.

Originally Posted By JohnParis: I need some help. Here is my set up (may be not tactically correct...) TA11, flat top, 11’5 barrel (1/9) and 55gr FMJ, mostly .223. What will be the best zeroing distance? How to use my BDC? Every advice, including another choice of ammunition, will be appreciated!

All you need to figure this out is a ballsitics calculator and some basic information (muzzle velocity, height over bore, weight of bullet and ballistic coefficient).

Here is a link to a good online ballistics calculator.
www.norma.cc/htm_files/javapagee.htm

You can probably find the average velocity for 55gr from an 11" barrel at The Maryland AR15 website. The rest of the information you need you can pick up from reading this thread. Let us know what you find out.

So how do you range find at those distances with the ACOG. I mean how do you know your target is at 500 meters etc?

.

How far off is a TA11, when used on a FAL with surplus ammo, say Port or Aussie? I'm trying to figure out if I should sell it and get a TA11C -

just in case

Where does the TA31RCO-M4CP fit into this equation? Do you know if it is compensated for 62gr? Why does it say 16" on one page of the Trijicon site and 14.5" on another, is the difference that small?

Product Link:
www.trijicon.com/user/parts/products1.cfm?PartID=481&back_row=1&categoryID=3

did I i miss this or is it not there - a TA01 on a 14.5/16 inch carry handle firing M193.... ? (I see there's 855, but not 193)

Thanks for the repost, Lumpy! I had almost forgotten about this post. I hadn't referred to it since I first installed my ACOG a while back.