ACOG + 77 grain

[LibertyCola]

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.

[Iraqgunz]

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.


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.

[Robb Jensen]

The 77gr ammo is much closer to .308 (M80 ball) trajectories.

[archad]

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.

[Skyyr]

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.

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.

[JSantoro]

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.

[ICANHITHIMMAN]

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

[JSGlock34]

My understanding is that the TA11C (.308 BDC as GotM4 said) is a good match for 77 grain from a 16" barrel.

[TehLlama]

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.

[Skyyr]

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.

[Gutshot John]

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.

[JSantoro]

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.

[JSantoro]

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.

Suggest trying 100m (109yds),if you can. I'd love to know your results.

[FromMyColdDeadHand]

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.

Could you PM me too with the cheat sheet?

IraqGunz- What is a 25M death drill? Google-fu came up with nothing.

[shootist~]

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.

[Skyyr]

Could you PM me too with the cheat sheet?


Done =)

[FromMyColdDeadHand]

Done =)

Thanks!

While we are here, how much does altitude affect these, like at 5000-6000 feet?

[Skyyr]

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).

[Skyyr]

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'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.

[shootist~]

Thanks!

While we are here, how much does altitude affect these, like at 5000-6000 feet?

Depends on the distance, but it's huge - somewhere in the ballpark of 5 or 6 MOA at 700M.

[Gutshot John]

Thanks!

While we are here, how much does altitude affect these, like at 5000-6000 feet?

You have to establish your baseline and go from there. My numbers for .308 state that zero shifts 1 moa for every 2500 ft in elevation change. So if you zero at 2500 feet of elevation you will probably shoot 1 moa low at sea level and 1 moa high at 5000 ft.

[Molon]

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.



Originally Posted by LibertyCola
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.






The difference in the trajectories of MK262 and M855 at 25 meters is miniscule; less than 0.25 inches (with a 200 meter zero). People seem to be confusing the difference between the trajectories of two loads versus the difference in the points of impact of two loads due to a difference in recoil vectors and barrel harmonics.


http://www.box.net/shared/static/obbtracj62.jpg

[556Cliff]

Molon,

Do you have a chart that compares the trajectories of M193, M855, 5.56 75gr TAP and MK262 out to 1000 yards or so?

[Molon]

Molon,

Do you have a chart that compares the trajectories of M193, M855, 5.56 75gr TAP and MK262 out to 1000 yards or so?

Here's one to 500 yards. MK262 would be extremely similar to the 5.56 TAP trajectory.




http://www.box.net/shared/static/7ptrckkcfd.jpg

[556Cliff]

Thanks Molon.

[edwin907]

Here is a chart for the TA11C and MK262. The TAP 75gr T2 5.56 loading is very similar to the MK262 ballistics as well.
Of course often what we see on paper doesn't always pan out. But in actual live fire use, at least out to the 600+ meters I've run it to on multiple trips, times, conditions, my TA11C's BDC has proven to be a superb match for the 75/77gr ammo out of my 16" Krieger barreled Recon.

The TA11C is a carry handle mount .308 BDC, and is a closer match than the flat top .308 BDCs, by about an inch per hundred yards.
Plus, it's a donut (red) which works great for BAC. The ACOG also performs well with the AN/PVS-22 UNS, no POI shift, very clear detailed image which inspires a lot of confidence in your ability to engage targets after darkness.

http://bellsouthpwp.net/e/d/edwin907/Bullet-Path.jpg

http://bellsouthpwp.net/e/d/edwin907/_2237132.jpg

[StuartBoyer]

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.

I know this is bringing back an old thread but it suits what I need to know. I am thinking of getting a TA31 for use on my 16" M&P for its i55 and 62gr ammo but would also like to use it on my other AR that uses 75gr 5.56 pressure ammo from a 16 inch barrel. Have you physicaly confirmed this sight in worked? I always heard sighting in 1.2 high at 100 in the TA31 models would make you off by 7 inches at 600.
I want any ACOG that would be a close match to 75/77gr ammo but hate the TA11C reticle which without adjustment is the closest match only ever being off 2 inches.

[shootist~]

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.

Obviously you used a 15.75" barrel at 230' elevation instead of the specified 14.62" at sea level. :sarcastic:

[archad]

I stand I corrected the data I posted was for a 20" flatop firing the 75 otm. Here is what I use for the 16" and my load 77 Nosler.
For a Flattop 16" firing 75gr OTM
Sight in 2.2" high at 100. You'll be dead on at 200 and 300; and MOA out to 600. This from arfcom thread. I never have put it on paper @ 600 meters but I can hit IPSC steel consistently in good conditions.

[Alaskapopo]

From the other site.




62gr. 5.56mm Flattop BDC

ACOGS that use this BDC are the: TA01NSN, TA31F, TA11F, TA11F-A
• This BDC is based on a 14.5" barrel firing M855 with the ACOG mounted to the receiver via a TA51 mount. If you have one of these, sight in according to manual.
• Stats used to figure BDC baseline: M855 traveling at 2,970fps with a BC of .304 and a height over bore of 2.5"
• For a Carry Handle 20" firing either M193 or M855 - you're screwed. There is no way you will match the BDC close enough to make it useful. You'll just have to learn where the rounds strike on your BDC and adjust.



For a Carry Handle 20" firing 75gr OTM @2750fps
Sight in at 100 per manual. You'll be very close to the BDC with the maximum deviation being 4" high at 600

For a Flattop 20" firing 75gr OTM
Sight in at 100 per manual. You'll be on the low side of the BDC instead of the high side but within 2" or less all the way out to 600.

For a Carry Handle 16" firing 75gr OTM
Sight in 0.4" at 100 (or sight in normally at 75). You'll be a half inch high at 200, dead on at 300 and within MOA to 600.

For a Flattop 16" firing 75gr OTM
Sight in 1.1" high at 100. You'll be 1" high at 200, dead on at 300 and MOA until 600 where you will be 7" lower than the BDC.

[archad]

Keep in mind the TA01NSN type BDC's are different than the TA01/TA31.

[StuartBoyer]

I stand I corrected the data I posted was for a 20" flatop firing the 75 otm. Here is what I use for the 16" and my load 77 Nosler.
For a Flattop 16" firing 75gr OTM
Sight in 2.2" high at 100. You'll be dead on at 200 and 300; and MOA out to 600. This from arfcom thread. I never have put it on paper @ 600 meters but I can hit IPSC steel consistently in good conditions.

Okay thanks for the update.
I think I may get a TA31 then rather than a TA11C. I can live with the 2.2" high at 100 and the MOA till 600 is a big plus, I plan to use this for 200+ meters anyways as I have an 11.5 ATAC SBR for close in work. This way I can also put it on my 16" M&P if I feel a need.
The rifle I wanna put it on has a poly 16" barrel that gets the velocity of a standard 18" and using my 75gr loads shoots has shot a .236" 5 shot group so I was hoping to find an ACOG I liked that would work with her.

PS
For anyone wondering this is what info Trijicon sent me for the drops used on their flattop .308 ACOGS and their 5.56mm 62gr ACOGs in MOAs:
"here are the general Bullet drops for the models you are requesting. All measurements are based off the 100m zero and given in MOA
For the .308
0
2.2 (Excluding the TA11J-308, No 200m mark)
4.7
5.5
8.3
12.4
17
22.3
28.4

For the .223
0
2 (Excluding TA11H and TA11J, No 200m Mark)
4.1
7.8
12.6
18.6
26.4
35.8

[ASH556]

Would the TA-11H-308 also follow the mk262 ballistics? Trijicon states it is calibrated for 308 flattop vs the TA-11C, which was recommended above and is calibrated for 308 carry handles.