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Posted: 2/29/2012 6:37:05 AM EDT
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I am ordering a scope with a custom BDC, I have found several different tables listing the drops on the web with some differences between them. I need to know which one, if either, is correct.
The firearm is a AR with a 14.5" barrel. Table 1: 100 yards - 0 200 yards - 2.8 300 yards - 12 400 yards - 29.5 Table 2: 100 yards - 0 200 yards - 2.3 300 yards - 10.7 400 yards - 26.7 |
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Quoted:
I am ordering a scope with a custom BDC, I have found several different tables listing the drops on the web with some differences between them. I need to know which one, if either, is correct. The firearm is a AR with a 14.5" barrel. Table 1: 100 yards - 0 200 yards - 2.8 300 yards - 12 400 yards - 29.5 Table 2: 100 yards - 0 200 yards - 2.3 300 yards - 10.7 400 yards - 26.7 You check here? |
| with those drops your talking about diffferences in density altitude, temperature, height over bore, muzzle V, what math is being used in the algorithm etc. The only way to really get reliable BDC is to shoot your ammo out of your gun, theres just to many variables to be able to pull the numbers off a table somewhere. And if you do those are about the margin of error I would expect if no other data is included. |
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Quoted: I am looking for a "average" BDC, like what is used on ACOGs. This is not for a precision rifle but for a combat carbine. In that case, average the numbers and be done with it. Do you really think +/- 1 inch at 400 yards will be noticeable at 400 yards for a combat carbine? |
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Quoted: I know it is not really going to make a difference but if I am going to custom order it I would rather have it correct. Like other people have mentioned - that kind of deviation could simply be due to air pressure, temperature or something else. Double-check a few more sites, but don't overly sweat the exactness of the numbers. |
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Quoted:
with those drops your talking about diffferences in density altitude, temperature, height over bore, muzzle V, what math is being used in the algorithm etc. The only way to really get reliable BDC is to shoot your ammo out of your gun, theres just to many variables to be able to pull the numbers off a table somewhere. And if you do those are about the margin of error I would expect if no other data is included. There is your answer. |
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You're gonna have to find out yourself for your specific gun. If you really want to go custom you'll have to do the leg work, then consistently use the same round.
Otherwise average and be close. The last ballistic chart I looked at was very close to the 29.5, but that was for a 16", so that's probably useless. |
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Here you go. You will need to measure your sight height above the bore and you will need to either measure your velocity with a chrono or guess from what others have come up with. This site will calculate the BC for your bullet type on the left of the page for you. Just plug the parameters in and it will probably be close, but you will still need to shoot at different distances to know for sure.
Personally, I just use an Aimpoint and zero at 300 and the trajectory is approximately 5" high at 150 and 12" low at 400 with my ammo. I don't have time to use come ups for this type of use. I am confused enough with just one dot. 
http://www.handloads.com/calc/ |
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IIRC, the M855 BC is approx. .271 depending on velocity and temp. of course.
Approximate: LOAD DETAILS: 5.56 M855 62gn Green Tip Penetrator Bullet Weight (grains): 62 Muzzle Velocity (FPS): 3025 Ballistic Coefficient: 0.271 Sight Height (inches): 2.5 (Typical AR Height) SIGHT SETTINGS: -1.48 Inches Low at 25 yards for a 100 Yard Zero Range –––- Velocity –––––– Energy –––––––– Path –––––––––– Drop ––––––––– Drift –––––––––– TOF ––––––––– Zero Adj 0 ––––/––––– 3025 –––––––– 988 –––––––– -2.50 ––––––––– 0.00 ––––––––– 0.00 –––––––––– 0.00 ––––––––––––– 0.0 100 –––/–––– 2685 –––––––– 992 –––––––– -0.01 –––––––– -2.06 ––––––––– 1.08 –––––––––– 0.11 ––––––––––––– 0.0 200 –––/–––– 2364 –––––––– 769 –––––––– -2.38 –––––––– -8.97 ––––––––– 4.60 –––––––––– 0.23 ––––––––––––– 4.5 300 –––/–––– 2061 –––––––– 585 –––––––- -11.25 ––––––– -22.45 –––––––– 11.50 ––––––––– 0.37 ––––––––––––– 14.4 400 –––/–––– 1786 –––––––– 439 –––––––- -28.31 ––––––– -44.03 –––––––– 21.61 ––––––––– 0.52 ––––––––––––– 27.0 500 –––/–––– 1539 –––––––– 326 –––––––- -56.40 ––––––– -76.64 –––––––– 36.05 ––––––––– 0.70 ––––––––––––– 43.1 ATMOSPHERIC CONDITIONS: Altitude (Feet): 550 Temperature (Degrees F): 59 Barometric Pressure (In Hg): 30.10 Wind Speed/Direction: 10 mph from E Humidity: 78 Hope this helps. |
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Brother if the Acog is your goal your already way way ahead of the game either way. My acog is designed exactly for the setup I have it mounted on and the ammo I put through it. Its a combat optic, its not designed to be what you would consider especially quote accurate by any stretch.
What it is designed to do, and what it does very well it allows you to very very quickly move a 19" 'target' until it fits perfectly between the stadia which instantly both ranges and centers your target for that elevation at which point you start pulling the trigger. It can also be used to guestimate where to lead a 'walking' target 'running' target etc. at different distances with a litle bit of practice, that's it. When someone is shooting back, minute of man is all that matters. BDC is similar in that you really cant cheat the math to much either. There are so many factors that are way beyond anyones control youll never even be able to tell which side of the table you might be off by. My advice is if your having the dials made up through Leupy or any other major glass maker. Just give them your stats and ask them for the correct dials. Thats probably gonna be your best bet anyway, with the added bonus of peace of mind over trying to guess which numbers are better. Theres a reason why Scout Snipers have a 4thousand dollar optic with crazy reticles, a mildot master, a range card, a laser rangefinder, a kestrel, and then a spotter just to make up for the kind of difference in spacing that your talking about. simply looking at a BDC and twisting the dialwont do it. It will get you close, but not wings off a fly. But then again neither will your or I anyway, and neither with m855, and neither will your rifle and so on and so on. |
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locations of the ARMY M150/RCO ACOG Stadia lines are as follows:
Range MOA(ref to 300m zero) 100 5.2 200 3.1 300 0.0 400 -3.4 500 -7.9 600 -14.2 700 -22.6 800 -32.3 (Positive means above 300m zero, negative means below) These are not drop values for M855 these are the measure locations of the stadia lines in MOA As others have stated i will reitterate. The difference between this value, the actual drop values, other ballistic drop calculations are only valid at one set of conditions. The solution in the ACOG is based on Firing tables which use the average 70ºF muzzle velocity and 59ºF air density at sea level. Any deviation from those conditions will introduce error. If you are going to be shooting at a specific altitude and know the muzzle velocity of your load through your barrel and know your BC pretty well over the flight range you intend on using you can calculate a nominal drop with many ballistic calculators that will be just fine (JBM is likely best if you dont want to buy anything). This will get you close to your standard set of conditions. Otherwise use the standard drop values as they are going to be as good as any solution, the BDC is a reference. |
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