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[ARCHIVED THREAD] - The Big ACOG BDC Chart (Page 1 of 2)
Posted: 3/6/2003 7:27:54 AM EDT
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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. |
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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. |
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Brouhaha and I compared his Blue box 75 grain ammo vs my Red box 75 grain ammo from his new 16" Colt barrel with 1/7 twist. Brouhaha contacted Black Hills and they said the Blue box was a bad batch and the Red box was loaded to NATO specs. Further testing needs to be done. But this was interesting none the less. High, Low, Average, Extreme Spread, Standard Deviation 2597, 2507, [b]2569[/b], 90, 30 Black Hills 75gr Blue Box Lot 0601120063 2569 2575 2585 2597 2589 2589 2584 2507 2528 2589 2772, 2704, [b]2737[/b], 68, 32 Black Hills 75gr Red Box Lot 1214032830 2710 2708 2704 2709 2772 2772 2751 2751 2771 2771 2569 vs. 2737 is a HUGE difference. 2737 is SMOKIN from a 16" barrel. I think thats going to be the ammo I keep in the magazine of my trunk gun. [;)] If any one else can chrono some rounds this would help and this should be made part of the Trijicon ACOG FAQ. |
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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. |
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No jackassery intended but is the POA/POI on the TA31F the tip of the chevron at 100m? BTW 16" midlength flattop. The shitty ammo deity smiled and I got 1.6" 5 shot at 100m using m885, was clanging away at gongers 100-300 but then shifted out and have no idea where it went at 400. |
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I used 2615fps as the velocity for the 16" 75gr. That is based on Hornady's data on the Bushmaster LE website. Apparently DevL and Brouhaha got hold of some of the souped up 75gr red box from Black Hills and they were reporting velocities closer to 2750fps (what I used for the 20") out of a 16" barrel. |
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Here is a copy of Jeff Hoffman’s (Owner of Black Hills) comments about the velocity differences posted above. Black Hills is devoted to manufacturing the Best Ammunition available on today’s market. They are shooters of their own products themselves and do extensive testing with each lot of ammunition produced. Jeff does most of the testing personally. This type of "Hands On" Experience is hardly found in today’s marketplace. Here are his comments. Tommy GPSS [size=6] The question about velocity differences between Black Hills Ammunition new And remanufactured ammunition seems to be generating quite a bit of interest And I thought I should address it now. I plan to investigate further after I Can acquire samples of the ammunition that was tested in the initial report, But for now here's my thoughts. We do NOT have different velocity specs for new vs. reman 223 ammo. We DO have different specs for military vs. commercial ammo, but that does not appear to be part of the issue here. I looked up our test records on the two lot#s that were provided for the new and reman ammo cited. The two lots were produced at different times. The remanufactured "blue box" was produced in 2003, the new "red box" ammo was produced in 2000. Our records from a 24" test barrel showed that when tested at time of production the velocities produced by each were: reman "blue box"-2749fps, new "red box"-2826fps , for a difference of 77 fps. That lot # of new ammo was somewhat high in velocity compared to where we normally run, but well within pressure. The reman ammo is right on spec. Our uniformity requirements, lot to lot, are better than what is indicated here. This degree of difference in our ammo is unusual,undesireable, and certainly not intentional. The difference in velocity, while outside our normal production standards, does not affect safety, function or performance, except for long-range trajectory difference. That is not to minimize the difference reported, but to clarify the effect. We will redouble our efforts to continue providing the best Quality ammunition available. Thanks to all who provided the input and who Use our ammo. As I mentioned above, I will continue testing of this Particular lot of "red box" ammo when I get samples. Jeff Hoffman [/size=6] |
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Quoted: I dont know the velocity, but the .308s are set for 168 grain bullets. Could you run figures for a TA11C with 18 and 16 inch barrels? I want to buy an AR10, but have not decided on length or optics, and I want to know if how this one goes. From memory......I believe the TA11C is good out to about 400 yds but believe the TA11D might be a better choice for .308 ?? (I seem to remember that Dave at Quantico Arms weighed in on this scope in another forum.) |
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THIS PUPPY IS NOW LISTED IN MY FAVORITIES. Of course I already knew all this data - especially on my TA-11C which is going to sit on my AR-10(TC) !! (Yeah, right sure I didn't !!) You people continue to amaze me with your real, practical knowledge. Oh, and THANKS !! |
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Explain. Isn't the 20" carry handle the most common setup? You are saying that the ACOG BDC totally fails to work with this setup? M855/M193 both of them? [b]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. [/b] |
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Explain. Isn't the 20" carry handle the most common setup? You are saying that the ACOG BDC totally fails to work with this setup? M855/M193 both of them? No, that is not what I am saying. The ACOG has at least three different BDCs. I am saying that the ACOG BDC designed for the TA01NSN, TA31F, TA11F, and TA31F-A will not work on a 20" carry handle because the combination of an inch higher mount and extra velocity from 5.5" more barrel will make it impossible to even come close to the trajectory on that BDC when using M193 or M855. If you must mount to a 20" carry handle, use the ACOG BDC designed for a 20" carry handle (the standard ACOG BDC that most models have and the first one detailed in this thread). |
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Do you mind running the same calculations for the two reticles on ARMS SIR/#38 rail? Yeah, that's not going to happen; but I'll be happy to give you some help if you want to do it. The SIR/ARMS #38 only raises the optic about a quarter of an inch, so for most of the figures it doesn't really change much (see the notes about raising and lowering the mount and you'll be able to figure out the difference from that alone). |
| Bartholomew_Roberts helped me figure the BDC on my new set up.(FlatTop AR)TA-31 mounted to Arms#19S shooting 77gr.(2752fps) out of 20" barell.If you sight in at 1.3" high at 100m you will be with in 2" of BDC out to 300m.And 3" to 4" from 400m to 550m.You could sight it in a little higher at 100m (1.8"high)and be even closer to BDC the whole way out to 500m.But since I do most of my shooting within 300m this will work for me the best I belive.Thanks again! |
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As a side note, For a 20" rifle with TA31 Donut mounted on ARMS 19S and SIR sleeve (my DMR project) firing Hornady 75gr SAMMI will only need to zero at 50 yards instead of 100 yards. Your maximum deviation will be about 2" when bullet passes through 100 to 250 yards. When the bullet passes 300 yard mark, there will be about an inch of difference in PoI. Then you will be dead on from 325 to 500 yards (less than 1/3 MoA, if my calculations are correct). Another hooah for Donut :) |
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Hey Bart Loosen up those typing fingers, you got work to do. As soon as I get a 6.8 upper, I'll get to work on that. Let me know if you need my mailing address [:D]. In the meantime, if anybody needs help figuring it out, I'll be happy to help via IM or email as long as you post your results here so the rest of us can learn from it. |
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Quoted: Hey Bart Loosen up those typing fingers, you got work to do. As soon as I get a 6.8 upper, I'll get to work on that. Let me know if you need my mailing address [:D]. In the meantime, if anybody needs help figuring it out, I'll be happy to help via IM or email as long as you post your results here so the rest of us can learn from it. Newbie Moment, but... I assume BDC is bullet drop compensator? It'd be really handle if the definition was in teh post somewhere. Thanks :) |
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Please help with this confusion: "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. 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." It says that if I use the Fixed Carry Handle 20" designed ACOG, then I can mount it to my 14.5" Fixed Carry Handle Set-up and it will be within MOA out to 600 using M8555???? Please let me know which scopes are most compatible with this particular set-up: 14.5", Fixed Carry Handle. Also how were these numbers calculated- I'm a financial guy by trade and would love to run some numbers but don't know how Mr. Roberts got them. Thanks, Mark |
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It says that if I use the Fixed Carry Handle 20" designed ACOG, then I can mount it to my 14.5" Fixed Carry Handle Set-up and it will be within MOA out to 600 using M855? Yes. The 62gr doesn't lose momentum as fast as the M193 and has a better ballistic coefficient. So even though you are starting out slower with a heavier bullet and shorter barrel, you track within MOA of the 55gr 20" trajectory. Also how were these numbers calculated- I'm a financial guy by trade and would love to run some numbers but don't know how Mr. Roberts got them. These numbers were calculated using a ballistics calculator. There are several good online calculators such as this one (requires Java): [url]http://www.norma.cc/htm_files/javapagee.htm[/url] If you look at each post, you'll see the data I used to establish the baseline trajectory for that ACOG. You'll need the muzzle velocity, height over bore, weight of bullet and ballistic coefficient. You can find ballistic coefficient and barrel velocities for the different combos at [url]http://groups.msn.com/TheMarylandAR15ShootersSite/_homepage.msnw?pgmarket=en-us[/url]. Height over bore for a carry handle mount is 3.5" and height over bore for a ACOG on TA51 mounted to receiver is 2.5". Once I had jotted down the +/- for that ACOGs BDC trajectory, I would change the details to the combo I was using (14.5" barrel, 3.5" height over bore, and M855 for your case) and adjust my sight in distance until I got a good match with the BDC trajectory. In some cases, you just can't match the BDC trajectory. In others, you can match OK up close or far away; but not both. I tried to go with the best overall match here. One other thing to remember is that your ACOG is calibrated in meters, not yards (100m = 109yds, 200m = 218yds, etc.). You can calculate the trajectory in yards (the initial values here were done like this) and it will still work for you since you are just matching one trajectory arc to another. You are just changing where on the curve you measure; but you may see an extra inch or so of variation at various ranges since the curves don't match perfectly. Whatever you do, do it consistently - if you start with yards, use yards all the way through. If you start with meters, use meters all the way through. All I ask is that you share whatever combo you worked up here in this thread so someone else doesn't have to work it out and that if you find I made a mistake somewhere, you let everyone know in this thread so it can be corrected. PS My name here is a pseudonym, so no need to be formal. |
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Thank you very much for taking the time to explain how you got those figures. I will be taking a look and post any findings that are different onto this thread. I just didn't want to buy say the TA31A mount to the carry handle on my 14.5" upper then have it hit way off at different ranges. I don't really ever shoot past 500 yards unless using a scoped .30 cal, but should I get the right scope then I might be able to use the little carbine more effectively. Thank you once again, Mark |
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Can anyone tell me the height (top of rifle picatinny to centerline of objective) of a TA31 on either a TA51 mount or ARMS 19 mount? I'm trying to calculate the over-bore distance so I can pick between the TA31 or TA31F. (Just to clarify, over-bore is CL of objective to CL of bore, right?) I have a both a flat-top carbine (15" bbl + 1" brake), and an AR180B (18.5" bbl), and I'd like to be able to use the ACOG on either (I'm working on putting a picatinny rail on the 180B). The TA31F should work really well with the carbine, but I'm afraid that the over-bore distance on the 180B will end up being almost equivalent to a 20" AR15 carry handle mount...in which case: For a Carry Handle 20" firing either M193 or M855 - you're screwed. Thanks for all the great info thus far! --Otter |
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Thanks again Bartholomew Roberts for the summary ... now that I have a TA31 at last have gone over the data could not find in the excellent summary, did not find any direct listings for a carry handle BAC ACOG on a 20" flat top . The fun facts did list the one inch lower mounting will cause your round to strike 1" lower at 200, 2" lower at 300, 3" at 400, 4" at 500, 5" at 600. Thinking one could infer from the listing of 20" flat top with M855 being setup for 0.8 inch high, that a little upward bias could help with M193 on the 20" flat top. My short summary of your data is: TA31 on a M4 flat top with M193 or M855, sight in 1.5" high (or 3.5" above center of donut hole) at 100 m - close to past 500 meters TA31 on a 20" flat top is 0.8 inch high for M855 - close to past 500 meters (guessing M193 might be close, will try 1/2 ...). |
[ARCHIVED THREAD] - The Big ACOG BDC Chart (Page 1 of 2)
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