For optics that REQUIRE 100 METER zero HELP!!

[equilibrium]

Alright, I just purchased a nice used TA31RCO for $700(friend) and although I am happy with my aimpoint I am pretty stoked to be shooting the RCO again since I haven't played with one since I was active.
A lot of us only have access to ranges that are in yards. Which brings up the question of how to properly zero an optic (like a trijicon RCO) at a 100 yard range. I have been searching for hours and have not found a clear answer as of yet. Any help would be appreciated.

[tmanker]

Set up a target at 109 yards with your rangefinder. Is this question as simple as it sounds?

[equilibrium]

Dude this is exactly why the question will not be answered. I have obviously found out that 100 METERS = 109 YARDS. Now, that would be very easy to do but MY RANGE IS OUT TO 100 YARDS!!! I can not just go and move the berm of this companies range or shoot 9 yards behind everyone else.

[equilibrium]

I really do not want this thread to wind up like ALL of the others regarding this subject have turned out to be, which includes people bickering and no answers being developed. I have a serious question and hope that someone could answer it for me. What would the POI be at 100 yards for a 100m zero? That might make the question easier to answer.

[tmanker]

Tell me what rounds you are using and the barrel length and I will plug into the ballistic software and tell you where to sight in to put it on at 109 yards. Fair?

[equilibrium]

Tell me what rounds you are using and the barrel length and I will plug into the ballistic software and tell you where to sight in to put it on at 109 yards. Fair?

Dude, that's perfect, thank you. It's a 14.5" barrel using M193 ammo.

[tmanker]

Using standard temp, pressure and altitude and 3050 fps for standard M4 and M193 BC of 0.243 I come up with POI 0.08 inches high at 100 yards will put you on at 100m. Is this what you are looking for?

[equilibrium]

Yea...in a way. It just comes down to realizing that unless you have a 100m range there is no way of actually zeroing at 100 yards. Not even the best shooters can get .08 high ya know? Answered the question for sure, thank you. Ive never had this problem since everytime I shot the RCO was during qual and the range was in meters. It is seeming like the practicality of an ACOG is low for anyone who doesn't have access to a range in METERS. I am sure that if you zero at 100 YARDS than the POI wouldn't change much within 300 yards and maybe out to 400 (which is more than I plan on doing). It was just eating at me that I would have this BDC that isn't correct. I plan on using the ACOG as a hunting scope (feral hogs and deer) but I want to make sure that I can have correctly placed shots and not be all screwed up because I have a 100 yard zero and not a 100 meter zero.

[equilibrium]

Does that software have the ability to show the difference in a 100 yard zero vs 100 meter zero with same bullet and barrel?

[tmanker]

POI will be 0.20 low at 100m if zero distance is 100 yards. That's about as good as I can do right now.

[equilibrium]

Alright man thanks for all of your help. Ill continue my search and see if I can find a comparison of the trajectory patterns for the 100 meter vs 100 yard.

[GH41]

With 193 don't worry about it. You aren't going to kill any deer or hogs at 3-400 yards anyway! 556 was never intended to kill things at that range. Not animals that don't know what hurt is.

[tmanker]

Are you looking to see what the difference is at specific ranges (100 yd/m, 200 yd/m, 300 yd/m, etc...)?

[equilibrium]

Yea i hear ya! Thats why i stated that I wasn't intending to shoot very far. I just wanted to make sure that with 55grain (SP, FMJ, ETC.) that I wasn't going to be too far off on the BDC. I know a lot of people zero their ACOG at 100 yards and it is fine. I am assuming that within 200 yards the difference in trajectory will not even be noticeable unless you are a sub MOa shooter and same probably applies to the further distances. I was just obsessing over it because I get like that sometimes, but feel a lot better now.

[tmanker]

I sighted my TA31RCO (chevron) in at 100 yards and have shot coyotes at 300 yards several times using bdc. Great optic for that range. I don't shoot past 300 with that setup and reticle.

http://i69.photobucket.com/albums/i75/thebestofindica/360bda05a8071bc30bad8dde94ce4001.jpg

[equilibrium]

Are you looking to see what the difference is at specific ranges (100 yd/m, 200 yd/m, 300 yd/m, etc...)?


Yes that is exactly what my next step was going to be!! That would help me understand my trajectory after I zero at 100 yards.

[equilibrium]

I sighted my TA31RCO (chevron) in at 100 yards and have shot coyotes at 300 yards several times using bdc. Great optic for that range. I don't shoot past 300 with that setup and reticle.

I do not plan on going past 300 either. The only time I shot past 300 yards in my life was during the range qual. Good to know that you zero'd at 100 yards and were effective out to 300. I kind of figured that the trajectory difference in range/yards couldn't be outside of + or - 5 inches.

[tmanker]

Zero at 100 yards will put you -12.98 inches at 300 yards and -17.07 inches at 300m (328 yds).

[TehLlama]

The POI delta between 100yd and 109yd is going to be less than a click, so I wouldn't be concerned about that.

For ACOGs and similar BDC setups, it is possible to start off with a zero for 300m on the tree at 36yd - it's not great, but it gets you close very quickly.

[TF82]

I get that the range stops at 100 yds., but if there's a chance you could put up a target at about 23 yds., do you think these would help?
https://www.m4carbine.net/showthread.php?95226-Improved-AR15A2-A3-A4-amp-AR15-Carbine-Zero-Targets-50-200M-100M-amp-300M-Zeroes

[equilibrium]

The POI delta between 100yd and 109yd is going to be less than a click, so I wouldn't be concerned about that.

For ACOGs and similar BDC setups, it is possible to start off with a zero for 300m on the tree at 36yd - it's not great, but it gets you close very quickly.

Yea I wish I could do that, which is what we used to do in the Marine Corps. I am just so constricted by local ranges that it is hard to find a range past 200 yards or even a range with a 36 yd target. I am just going to have to perform the 100 yard zero and the BDC may be off a tad but that's alright. I'm not attempting to be a sub MOA shooter with this optic. I only purchased it because I have trained with it for years and feel comfortable with it.

[equilibrium]

I get that the range stops at 100 yds., but if there's a chance you could put up a target at about 23 yds., do you think these would help?
https://www.m4carbine.net/showthread.php?95226-Improved-AR15A2-A3-A4-amp-AR15-Carbine-Zero-Targets-50-200M-100M-amp-300M-Zeroes


They may help and I have thought about giving those a try at 25 yards and getting a nice group just a tad high in the 100M circle. Basically I would be trying to compensate for the meters to yards and get POI a little high for that purpose.

[Failure2Stop]

POA/POI at 100 yards is going to be just fine.
The bullet hangs at the line of sight for quite a while around 100 yards/meters, and will be within the adjustment capability of the optic.
The best thing to do is to fine-tune your elevation at long range when the opportunity presents itself.

[cbx]

Does your range not allow you to go beyond the 100 yd mark with your own little target stand? Go on a day with no one there and cheat back your end and the target end to get that last nine yards.

Our just don't worry about it and go for 100 yards even. It's a very small difference. Depending on your ammo used your may not be able to tell.

[equilibrium]

Does your range not allow you to go beyond the 100 yd mark with your own little target stand? Go on a day with no one there and cheat back your end and the target end to get that last nine yards.

Our just don't worry about it and go for 100 yards even. It's a very small difference. Depending on your ammo used your may not be able to tell.

The range does go to 200M so I could zero at the 100 yard range and make the adjustments at 200meters. That would hopefully help me get a better zero. Either way like others have said the difference isn't going to be insane and I am not precision shooting with an ACOG.

[Renegade]

Dude this is exactly why the question will not be answered. I have obviously found out that 100 METERS = 109 YARDS. Now, that would be very easy to do but MY RANGE IS OUT TO 100 YARDS!!! I can not just go and move the berm of this companies range or shoot 9 yards behind everyone else.

Just zero at 100 yards, The difference is like .2 MOA.

[Failure2Stop]

The range does go to 200M so I could zero at the 100 yard range and make the adjustments at 200meters. That would hopefully help me get a better zero. Either way like others have said the difference isn't going to be insane and I am not precision shooting with an ACOG.

If your COG has the chevron reticle, it will make your 200M shooting less than precise since the 200 is at the apex of the bottom part of the chevron.
http://i139.photobucket.com/albums/q319/amynruss/Technical1.jpg
This tends to make me hold a little high on my desired target, especially if the fiberoptic isn't taped off.

Wind-drift is also a bit more shakey at 200 since there isn't a readily referenceable wind-hold in the cog and lots of well-meaning folks wind up dialing in an atmospheric unwittingly.

[TehLlama]

If your COG has the chevron reticle, it will make your 200M shooting less than precise since the 200 is at the apex of the bottom part of the chevron.
http://i139.photobucket.com/albums/q319/amynruss/Technical1.jpg
This tends to make me hold a little high on my desired target, especially if the fiberoptic isn't taped off.

Wind-drift is also a bit more shakey at 200 since there isn't a readily referenceable wind-hold in the cog and lots of well-meaning folks wind up dialing in an atmospheric unwittingly.

The tendency to hold a bit high with them applies to me too, the TA31 reticle is one of the few I never bother to zero at 50 or 200m if I can help it (even on one of our Table I/II shoots somebody insisted on refining zeroes at 200yd for table 2, unsurprisingly myself and two guys in my fireteam who agreed we should leave it alone were three of the top four scores on that table 2). I'd literally rather zero the ACOG at 36yd and be off some vertically than be in doubt for both trying to make the crotch of chevron be the POA and tend to be off (aggravated by shooting position NPOA changes). When I get the chance, I'll zero for 100m in a partially supported sitting as that tends to produce ideal results for me. I'm not unwilling to make windage adjustments at 200yd/200m, but that's only in dead still wind conditions; for that type of optic if I'm on at 36yd it's close, and on at 100yd/100m is perfect... I'm much more likely to be a source of error beyond that because being so tight to the optic means that I can make a half millimeter move of cornea position and do more to affect POI relative to sight picture than making one-click adjustments (especially since forgetting to smack the turrets tight just once is plenty to throw the whole data set off).

[Zane1844]

Does your range go to 300 yards? If so, you can zero at 100 yards, then take it out 300 and make adjustments so that the 300m is zeroed at 300 yards. That should get you close are longer ranges.

Just saw that you said it goes out to 200yd.

[Failure2Stop]

Does your range go to 300 yards? If so, you can zero at 100 yards, then take it out 300 and make adjustments so that the 300m is zeroed at 300 yards. That should get you close are longer ranges.


I don't recommend this.
A 100 yard zero is closer to a 100 meter zero than a 300 yard zero is to a 300 meter zero for the ACOG BDC to be correct at longer range.

If you zero at 91.44 meters (100 yards), you will show around 15 inches of drop at 300 meters.
If you zero at 100 meters, you will show pretty much the same at 300 meters.
If you change to a 300 yard (274 meters) zero, your 100 meter zero will be off by 4 inches, and your 300 meter zero by about 3 inches. Not a huge deal on the 300 for most folks, but your 600 would be off by over 20 inches. Again, most don't actually shoot that far, but if I can avoid putting that much error into the optic I generally won't.
*The above based on M855 at 2850-2950 fps*

It seems counter-intuitive, but a 100 yard zero instead of a 100 meter zero will show no appreciable difference at 600 meters.

Still, you don't know exactly where the bullet is going to go until you shoot it at actual distance.

[equilibrium]

I don't recommend this.
A 100 yard zero is closer to a 100 meter zero than a 300 yard zero is to a 300 meter zero for the ACOG BDC to be correct at longer range.

If you zero at 91.44 meters (100 yards), you will show around 15 inches of drop at 300 meters.
If you zero at 100 meters, you will show pretty much the same at 300 meters.
If you change to a 300 yard (274 meters) zero, your 100 meter zero will be off by 4 inches, and your 300 meter zero by about 3 inches. Not a huge deal on the 300 for most folks, but your 600 would be off by over 20 inches. Again, most don't actually shoot that far, but if I can avoid putting that much error into the optic I generally won't.
*The above based on M855 at 2850-2950 fps*

It seems counter-intuitive, but a 100 yard zero instead of a 100 meter zero will show no appreciable difference at 600 meters.

Still, you don't know exactly where the bullet is going to go until you shoot it at actual distance.


Thanks for the insight. It is much appreciated and definitely what I was hoping to hear/find out. As long as the 100 yard zero isn't much different than the 100m zero than that is awesome.

[Zane1844]

I don't recommend this.
A 100 yard zero is closer to a 100 meter zero than a 300 yard zero is to a 300 meter zero for the ACOG BDC to be correct at longer range.

If you zero at 91.44 meters (100 yards), you will show around 15 inches of drop at 300 meters.
If you zero at 100 meters, you will show pretty much the same at 300 meters.
If you change to a 300 yard (274 meters) zero, your 100 meter zero will be off by 4 inches, and your 300 meter zero by about 3 inches. Not a huge deal on the 300 for most folks, but your 600 would be off by over 20 inches. Again, most don't actually shoot that far, but if I can avoid putting that much error into the optic I generally won't.
*The above based on M855 at 2850-2950 fps*

It seems counter-intuitive, but a 100 yard zero instead of a 100 meter zero will show no appreciable difference at 600 meters.

Still, you don't know exactly where the bullet is going to go until you shoot it at actual distance.

Thanks!

[TehLlama]

Thanks for the insight. It is much appreciated and definitely what I was hoping to hear/find out. As long as the 100 yard zero isn't much different than the 100m zero than that is awesome.

You'd need a world class shooter on a high end bughole grouping benchrest rifle with its favorite load just to produce 10rd groups that could actually have a statistically significant mean POI different between the two... and even then that difference in impact location is probably about 1.5 bullet radii at 100yd.
Most 5.56 rounds (and virtually all BDC compatible rounds like 62gr M855 or similar) are really impressively flat at 90yd-110m if you're zeroed for 100yd/100m, so I'd leverage that as much as possible. Helpfully that's also one of the better parts of the reticle design to line up consistently, so the tiny bit of error from having the target be a touch further away (328ft) is really tiny since that bullet traveling (still traveling about 2650-2700fps in F2S's example) is basically at the top of its arc anyway during that time. More importantly, since each click is good for 0.5" at 100yd...well...
Very long way of saying - the 100yd zero being 'much' different than a 100m zero isn't much of a question when they're within less than ONE click of elevation from each other when using any load that fits the BDC reticle.

ACOGs are just an optic I file under: zero it as directed at 100 unless you have to go with less, then just run 36yd for 300. The units are left out, because they're really not important - I don't have anything that shoots unbelievable groups with 62gr M855/M855A1/62grTTSX/64grSP or similar ammunition, I'm still well within the error cone of that ammunition and barrel, and TBH anything with the kind of accuracy potential to care about the 100yd-100m difference is something I don't want to put a low-ish powered fixed optic anyway - something that precise and I'll want to put the closest facsimile to the hubble space telescope that fits on there and I want to shoot beautiful groups with heavier match bullets at distance. For something I want to bolt an ACOG to, being within .5MOA on POA to ideal external ballistics is still going to put me right in the middle of the best-case 1.5MOA combination I'll get out of a barrel that happens to like one particular 60-64gr match load (and hilariously well within the 4MOA accuracy standard of M855 stuff).

[Nick S]

Zero it at 100 yds and don't worry about it. Or more importantly shoot it after its zeroed at longer ranges and reconcile the subtensions on the BDC if you have access. If you don't its a moot point anyway.

tmanker provided you with data from a ballistic computer but you need to realize there are a lot of variables that will effect the data. So what Im saying is shoot it and don't get too wrapped up around the axle.