First of all, Mods feel free to move me if this is the wrong forum. I just figured that these guys would probably have the most technical knowledge.
So sometimes I get bit by a curiosity bug and it causes me to research and research and dive deeper and deeper until i’m satisfied (thats what she said). The most recent case has developed as a result of me trying to pick what ammo to buy/test for my new platform (SR-15 with TA31 ACOG). I know its not a tack driving setup but why not pick the closest/best match possible, right? Which leads me to my problem:
Trijicon states that the BDC on the TA31 is calibrated using a 62 grain bullet @ 3050 FPS. I have not been able to find any other information. As you can see that leaves it kind of vague. Whats the BC? Fired at what elevation, temp, humidity, etc. Again, I know this is by NO MEANS a precision/long range optic. But again - why not utilize it as best as possible (without forcing myself to use 1 kind of ball ammo).
Now the question:
Does anyone know A) what bullet they used to calibrate this OR even better B) the drop distance of each BDC hash mark. Because thats all that really matters. If I could know that the “5” hash mark = 65" drop @ 500 yards thats all i’d need.
Anyways, I know its a long shot but figured id ask anyways.
Why? I just figured it out and learned something in the process. What I was describing are called the ‘subtensions’. So from the 100 yard Chevron tip to the 600 BDC hash is a subtension of 19.2. Multiply this by 6 and you have a drop of 115". No matter what. That mark is 115" below my zero (@ 600 yards).
Now I can play around with BCs, velocity, elevation, temp, etc etc and find/create a round that matches. Turns out for my environment the Gold Dot 62 grain is a very very close match to all those subtensions and is also a terrific round. Now I can shoot from 100-800 with instantly and without touching a thing.
I thought I had read on Trijicon’s website that it was calibrated using M855 out of a 14.5" barrel with distances marked in meters. I have one and it’s mounted on a 14.5" SOCOM profile Colt barrel.
I think I still have data from that shoot somewhere (~5 years ago). I do remember it wasn’t on at 200 & I compromised the 100 & 200 poi to get in the black at 300, 400 & 500. We were shooting AQT that day & I put most into the black.
I just used this same ACOG again last month on a military base (CAJMTC) shooting the computer controlled pop up Ivan targets. Range was 50 - 300 yards. We were told by .mil that we couldn’t shoot using M855 for some reason. That’s exactly what they were handing out to the troops shooting before us though. I left the M855 at home. I didn’t have any load data using PPU 69 or 75 grain but that’s what I shot. Ivans went down so I must have been hitting them. Unfortunately, the scoring computer took a crap and I didn’t get a hit total.
So it did exactly what it was supposed to do even without a calibrated load. The TA31 allowed me to quickly range and hit unknown distance targets that popped up. The subtension widths match a “shoulder to shoulder” man sized target at the designated distances.
From a Trijicon CS rep last year when I inquired about my TA31F;
*subtensions are in MOA
Good morning and thank you for your inquiry, I have included the most popular ACOG and I have included subtensions to view. The TA31F-G was designed using a 16 inch barrel with 62 grain XM855 ammo getting about 2800 fps with a HOB of 2.83. I do not have the BC listed sorry.
TA31F-G 62grain
Tip Of Chevron is your 100 Meter marker.
Tip of chevron to 200 cut out/underside of chevron point = 2.0
I understand mils and subtensions. I’m not interested in your math. Everything you’ve posted convinces me that the military chose wisely when they chose mils. Thanks anyway.
