Since I believe the ACOG’s distance calibration is for 62 grain how much of a differece would there be between the 62 grain trajectory and 77 grain?
edit: this should probably be moved to the terminal ballistics section, my bad.
While doing some 25M death drills, I discovered that there was a big difference between the 62gr. M855 and the Mk 262 MOD1 ammo that I had. I believe that it was about 2 feet higher than the M855. Unfortunately we did not have a longer distance range available.
The 77gr ammo is much closer to .308 (M80 ball) trajectories.
I used this data for my 16" 5.56 pressure 77gr and a TA31
For a Flattop 16" firing Nolser 77gr OTM @2700fps
Sight in at 1.2" high at 100. You’ll be 0.5" high at 200, 1.5" low at 300 and within MOA to 600.
It’s significant enough that most experienced shooters get the 7.62/.308 caibrated ACOGs, as the BDC almost perfectly matches the 77gr. round’s trajectory.
TOS has some good in-depth posts about 77gr. and .308 ACOGs.
The above poster’s advice is sound; if you’re stuck with a 5.56 ACOG, then sight in 1" (1.1" according to Bartholomew Roberts) high @ 100yds to get the BDC to match as close as possible.
Thanks, guys, you just gave me something to play with in a couple weeks! 
I’ve known that Mk262 flies high since 2006, but keep forgetting to hit up the boss to experiment w/the RCO so we can see about maybe getting something published. I needed the reminder.
Sharpshoot me on this, please, somebody. My cheat sheets are at the office (and I’ll be doing a Search),
From a 14.5" 1in7 barrel, BC of .362 (or is it .340?) @2650fps. Y/N?
What’s the MV from a 20" barrel? I’d like to see how an M4RCO (taller BDC) on an M16A4 matches up…or doesn’t, as the case may be. EDIT – I see Molon putting it @2830fps in Post #49 on Page 3 of this thread: https://www.m4carbine.net/showthread.php?t=26127&highlight=mk262&page=3. I’m gonna go with this unless somebody had better info than one of our more cagey Tribal Elders.
This is going to be good info I was not aware of the TOS post I did a search no joy. I have been wondering this for quit some time but having sold my ACOGs I cant do my own testing.
I am wondering though with the MK262 becoming more and more prevelant in the field why has TRIJICON not published refined DATA for troops who are issued MK262 ammo. I have not seen anythig come out of Quantico or Fort Benning either.
It only makes sence to publish DATA for the 77g OTM.
Jon
My understanding is that the TA11C (.308 BDC as GotM4 said) is a good match for 77 grain from a 16" barrel.
JSaontoro (and others) - would this be a practical goal?
Try and figure out what distance the 5.56 BDC subtensions (the original 300/400/500/600 hash marks) would correspond to with 77gr, and verify how close those are? I’m looking at tables of ballistics info, but I’m not making headway reconciling the arc in mils to the actual drop… aghh.
I think it would be easier to simply just use the original intended distances. The BDC’s will never match, so you have to simply have to set your zero so that the overall difference between arcs is as small as possible. The ACOG isn’t a precision optic - it’s a combat optic, so an inch or two of difference between BDC’s isn’t a huge deal.
Check your PM’s - I PM’d you a quick cheat-sheet from TOS for rough zero points with 75/77gr ammo.
With any optic/ammo/gun you’re going to have to get your dope. I also went with a .308 TA-33 using Mk262. I only tried it at a hundred yards but it was pretty close. I’m going to shoot a bit more this weekend and see.
There is probably a lot of good data available but your individual conditions may vary.
I’m more concerned with being able to specifically articulate “You gots 262? And zeroed with M855, rite? AmIrite? Then, if you wanna take a 400m shot, hold HERE on the BDC.”
I got one of my many Page 11 entries arguing with my unit’s 1stSgt about a related topic. We got Mk262, had never seen it before, but some of us Sgt-and-below enlisted scum could tell by looking at the thing that it was different and would print different with out RCOs than M855. We were confirming our zeros (done with M855) @ 33m , and the groups were significantly higher. Everybody’s groups. We brought up the idea that maybe we needed to stick with M855 until we got more info about the new round. Because we were Sgt-and-below enlisted scum, we were Wrong (note the capital W), and the Marines were simply both not applying the fundamentals and didn’t know how to use their optics because we’d failed to train them correctly. Riiiiiiiiight.
Using the phrase “can’t-teach-me-nuttin’ lifer d**khead” at least got me in front of somebody senior enough to listen and countermand that Masters of the Universe idiot. Worth it at twice the price. We’d still be there, trying to zero, otherwise. Not a good use of resources.
All that to lead up to this idea: We already have people zeroing RCOs incorrectly with the ammo the thing is tuned for. Specifically, doing the 33m groupings without doing the 100m confirmation, because, to them, the 33m thing is in The Book, and therefore is an acceptable shortcut to having to do the work necessary to get a 100m zero done as it should be. I have to presume that the same sort of oxygen thieves as my aforementioned 1stSgt will follow the same book with Mk262 ammo, regardless of evidence that that won’t work, and make an even bigger hash of it…unless we can get a new The Book made, if for no other reason than to have something to point at and fruitlessly say “SEE!!!”
I can speak to a different zeroing process to certain entities because they have senior people with intact brain stems, so I love what’s already been posted, and will beg, plead, and cajole (meaning throw a tantrum and hold my breath) to see about shooting the piss out of it to ensure that it works. I’ll do it myself out of my own pocket if I have to. Sounds fun!
For the ones who think along the lines of “I’ve been Marine-er-ing for 4,218 years and have Seen It All…,” and will continue to misinterpret/incorrectly utilize the already-established procedure, my desired endstate is to be able to articulate what the results of that will be, and how to mitigate it so that the weapons are still reasonably usable. If I can point to a blow-up of the reticle pattern and state (see first sentence, above)…
…that’d be a real good start.
Suggest trying 100m (109yds),if you can. I’d love to know your results.
Could you PM me too with the cheat sheet?
IraqGunz- What is a 25M death drill? Google-fu came up with nothing.
A .308 reticle makes the most sense. I used a Mark 4 1.5-5 CQ/T with .308 turrets for some load development with 75 grain bullets. It was reasonably close out to 500 meters with a 16" AR. At distance, it will be a matter of knowing your hold points on the ACOG.
Done =)
Thanks!
While we are here, how much does altitude affect these, like at 5000-6000 feet?
Unfortunately, since each bullet has a different ballistic coefficient, and density altitude (the actual altitude value you’d use) has more affecting it than simply altitude itself, there’s no way to tell without either 1) plugging in the numbers with regard to the air density and BC or 2) simply shooting and seeing. If the BC’s were the same for both the round used for the BDC, then it would simply be a matter of adjusting for altitude. Since it’s not, that complicates things, hence the required calculation. Any zero’ing guidelines typically apply to a standard day at sea level (unless otherwise noted).
I’m assuming that while you guys are using 262 that you won’t be using M855 interchangeably during the same range time or loading alternating magazines of different ammo.
Assuming the above is true, then I would simply calculate the average elevation click difference between the two rounds and have your guys simply adjust upon changing the load, and then adjust back when using M855.
While I can appreciate that you’d like to teach them to critically think and adjust the BDC to their current load, I would think it would simply be more of a liability to have them memorize arbitrary BDC values to replace the solid 300m, 400m, and so on, then it would to have a solid rule of “8 clicks elevation up, verify, set.” It would also aid them in being faster on target and there wouldn’t be any lag time going back to M855, aside from readjusting the elevation X number of clicks.
Just my humble .02.
Depends on the distance, but it’s huge - somewhere in the ballpark of 5 or 6 MOA at 700M.