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Thread: Question on TA31RCO-A4

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    Question on TA31RCO-A4

    I have a question I need help with about my TA31RCO-A4 ACOG. Its mounted on a 20" 1:7 M16A4 Clone build I have and I have it zeroed as intended at 100m with Federal XM855. My zero group size was .475 MOA I am very pleased with the optic and barrel combo. Anyway I was playing with a ballistic calculator (strelok pro) on my ipad and it has a setting to see the bullet drops displayed with a TA31RCO-A4 Reticle and the ammo you punch in or choose from a list. Several XM855 loads are available to choose from. I did ran the calculations with XM855 and noticed that the aimpoints aren't exact ie 100m is 100m like it should be but the 300m aimpoint is really 295m, 400m is 392m, 500 is 489, 600 is 588, 700 is 692 and 800 is 799. I know the aimpoints probably aren't 100% correct but shouldn't the ACOG be setup so that the 600 meter aimpoint isn't 12 meters off? Is that a little bit too big of an offset at that distance considering this ACOG was specificly built for this round? I can see a few meter (5 of so) off at each aimpoint but 12 meters seems a lot especially at that range. I asked trijicon about it and they said they can not tell me what spec the M855 load its calibrated to nor can they answer any questions about this optic period, I found that response weird so I emailed them again and got the same response. And yes before I get asked I do have OCD with some things.

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    Considering that in different environments (high altitude, hot / cold weather), the BDC will be even more off.

    With BDCs, different ammunition, barrel lengths, configurations, etc it is going to be measuring with a micrometer and cutting with a chainsaw.

    Try shooting it at 600m at tell us what your real work results look like.

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    Quote Originally Posted by RHINOWSO View Post
    Considering that in different environments (high altitude, hot / cold weather), the BDC will be even more off.

    With BDCs, different ammunition, barrel lengths, configurations, etc it is going to be measuring with a micrometer and cutting with a chainsaw.

    Try shooting it at 600m at tell us what your real work results look like.
    Wish I could. Longest range around here (several counties) is only 300 yards. I dont need it to be exact I was just wondering if this is normal. Don't wanna have something weird going on with the optic or gun. I figure as much as I have in the gun that Murphy's law got me.
    Last edited by StuartBoyer; 03-17-18 at 10:35.

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    If you want precision at those distant ranges, ACOG wouldn't be the optimal optic for that role.

    It's a combat optic.

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    Quote Originally Posted by RHINOWSO View Post
    If you want precision at those distant ranges, ACOG wouldn't be the optimal optic for that role.

    It's a combat optic.
    10-4 I get that, I got it for a clone build I was just curious if that much variation is in spec. I find it odd that its so close at some distances for example only off 1m at 800 and 5m off at 300 but be so far off at others. I have tried the ballistic calculations with several companies M855 loads (Federal, IMI, PMC,and GGG) and using a BC of .304 and .349 listed with some on them and the drops can even be as far off as 30m on some calculations using IMI at 300 mark and 62m off at 700. I am curious if in anyone's experience with this optic and load if they have had results that kinda matched the calculations or is it possible that the reticle info used in Strelok Pro to create a BDC image is using incorrect data? I expect it not to be "sniper exact" but even on a combat optic being 62m off seems extreme for that round, I would think it would make that holdover so far off that a com hit would have to be complete Kentucky windage. I probably could have phrased it better than my previous post but that's what I get typing after an 18 hour patrol, trying to type it out before I get side tracked and forget to ask. I appreciate all the replies though.

    Trijicon wouldn't even tell me if that much variation was normal or acceptable, Also I love how they say "zeroed from factory", I know it wont be true zeroed from factory (just mechanical) but I expected it to be on paper (20x20 target) . It was way off at the initial 25m rough zero test I did before going to 100, 34 clicks high and 34 right to be exact It barely hit my target stand at the one group I shot at 25 to get me roughly on paper for 100.
    Last edited by StuartBoyer; 03-17-18 at 12:07.

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    No worries, I know that companies can be frustrating sometimes with data / information.

    My gut feeling is that USMC told Trijicon the specs they wanted the TA31RCO-A4 to conform to - "Designed to the exact specifications of the United States Marine Corps, the unique reticle pattern provides quick target acquisition at close combat ranges while providing enhanced target identification and hit probability out to 800 meters utilizing the Bullet Drop Compensator."

    So it wouldn't surprise me if the Corps said "make it like this" and Trijicon made it like that. With NDAs, contracts, etc, Trijicon may not be able to provide more specific data.

    This is part of the reason I don't really like BDC reticles, because 'it depends' seems to be ever present. What round, what BC, what barrel length, what height over bore is the optic mounted, etc, etc.

    Granted I have limited time behind an ACOG and I'm sure it's good at being a rugged, easy to understand optic to get round on / near targets at longer ranges - because unless you know the exact distance to target, you are estimating distance with the reticle - which is yet another 'guesstimate' to add into all the other possible variables.

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    Quote Originally Posted by RHINOWSO View Post
    ...measuring with a micrometer and cutting with a chainsaw...
    Love this line. Like Rhino said, you have to shoot the distances to see how the BDC actually lines up (and it will be different with different ammo). There are just so many variables to take into consideration. Go shoot it at 100, 200, and 300 and see how that lines up with whatever load your using. My guess is it wont be that far off in practical application at those ranges. Try it before you get too worked up. Calculators are nice but the bullet is the truth.

    You may be able to use the actual data in your ballistic calculator to true-up the calls. Not sure if strelok has this feature or not. Shooter does.

    Also, something that caught my eye... sub .5 MOA grouping with XM855 is like a total miracle. That's just really hard to believe given the construction of that projectile. I'm not saying it's impossible but it is very likely an outlier and not a good predictor of future performance. I would run whatever test that was a few more times to confirm that data is truly repeatable and something you can rely on.

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    Quote Originally Posted by Bullz View Post
    Love this line. Like Rhino said, you have to shoot the distances to see how the BDC actually lines up (and it will be different with different ammo). There are just so many variables to take into consideration. Go shoot it at 100, 200, and 300 and see how that lines up with whatever load your using. My guess is it wont be that far off in practical application at those ranges. Try it before you get too worked up. Calculators are nice but the bullet is the truth.

    You may be able to use the actual data in your ballistic calculator to true-up the calls. Not sure if strelok has this feature or not. Shooter does.

    Also, something that caught my eye... sub .5 MOA grouping with XM855 is like a total miracle. That's just really hard to believe given the construction of that projectile. I'm not saying it's impossible but it is very likely an outlier and not a good predictor of future performance. I would run whatever test that was a few more times to confirm that data is truly repeatable and something you can rely on.
    Using the exact barrel and ammo specs that this ACOG is configured for, M855 in a 20" 1:7 chrome lined A2 profile barrel. Wish I could get a range past 300 yards to confirm further And only 202 feet above sea level, thats why it seems odd its so far off. Guess I shouldnt worry though since its not a duty gun, just a safe queen.
    Only target I kept from that day posted below, measured using the OnTarget program. Its some what repeatable but hard, I had gusting winds that day average 8.5 mph max was 19, I shot between times the wind died out or mostly down. Over all average is closer to about 1 MOA. That pic is the group it shot on final zeroing adjustment. Yeah it was a miracle as I haven't shot in a long time at range (mostly CQB shooting is what I do since I'm a cop, I have to provide my own duty rifle and ammo plus our range is only 25yards here so to shoot further I have to drive 30 minutes away to a club range that is only 300 yards out so I only make the trip a few time a year. )

    ACOG 100 by Stuart Boyer, on Flickr


    EDIT: I was incorrect checked my target tube and found another target I shot at 100m with the ACOG. Fire ten 3 shot groups over all before having to call it a day but I dont have them all saved unfortunately. Avg of what I do have by rough ruler measurement is indeed 1 MOA and worst was 3 MOA with flyer. Green Mountain 1:7 A2 chrome lined barrel using Federal XM855 from the 150 round bulk boxes.

    OFF TOPIC: I shoot rifles so little due to ammo cost but am way better with them than pistols which is why I tell my chief all the time I prefer to pull my rifle out on calls rather than my pistol. Still avg 90s in qualifications with pistol each year. We only get to shoot once a year (30 rounds) though due to cost and budget so all shooting I do outside of that has to be done on my time with my $ which sucks since I cant claim it on taxes and cops don't make a lot in small departments. Most of my gear is self provided so my funds got to pistol ammo and targets when I can. I am stocking up on rifle ammo now So I can attempt to get into a carbine class that is LE certified for my state and Ill have to self provide that too if I can get in class as it fills up and is only offered twice a year and first class is already full. I was the only officer in my department certified with rifle last year (had to go to another department to do that) but that's run out now and I cant find an instructor certified to sign of on me running the qual course this year.
    Last edited by StuartBoyer; 03-17-18 at 17:08.

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    I probably run more than 500 rounds through that same scope on a 20 inch barrel in the last few months. I would expect to see a bigger variation between different brands of ammo then how specific the callout is for on that scope. In my case, approximately 1600 feet above sea level, my 100 yard zero is right on at 435 yards with the exact ammo I'm shooting. I can all but guarantee you switching different brands of ammo would give me a different point of impact that would exceed the difference you are looking at.

    The intent of the scope is to increase the probability of hits on a man sized target. I would consider using the 300M hash mark at 300M and calling it good.
    Last edited by opngrnd; 03-17-18 at 17:06. Reason: Added info

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    Quote Originally Posted by taekwondopreacher View Post
    I probably run more than 500 rounds through that same scope on a 20 inch barrel in the last few months. I would expect to see a bigger variation between different brands of ammo then how specific the callout is for on that scope. In my case, approximately 1600 feet above sea level, my 100 yard zero is right on at 435 yards with the exact ammo I'm shooting. I can all but guarantee you switching different brands of ammo would give me a different point of impact that would exceed the difference you are looking at.

    The intent of the scope is to increase the probability of hits on a man sized target. I would consider using the 300M hash mark at 300M and calling it good.
    Not sure I can apply any of what your saying to my setup as your using the scope zeroed in yards not meters and also from what I assume ammo its not calibrated for which changes the BDC markings POI to not be consistent with its intended ranges. I am also assuming your referring to seeing what the BDC equals out to with the factory markings versus the load and zero selected by the user and making a self made BDC chart but that's not what I am referring to.
    Last edited by StuartBoyer; 03-17-18 at 17:54.

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