5.56mm M855 ball long range trajectory


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MTMilitiaman
November 15, 2010, 08:00 AM
So Leupold apparently came out with a new reticule called the CM-R2. It looks alot like the horseshoe dot ACOG reticule. I think it has a lot going for it and is very close to what I've been looking for in an optic for my M1A. I wish it was a FFP, but I won't nitpick too much. It has a fast CQB reticule with a quick, math-free range estimation BDC. Only problem is it is calibrated all the way out to 900 yards for the 5.56mm 62 gr M855 ball round. I thought it was ridiculous enough when they expected me to believe this round was effective at half that range, but whatever, I digress. Point is I need to know what the long range trajectory for the military ball round looks like.

I am trying to put this on an M1A. I've been asking for an optic like this calibrated for 7.62 for years, and I think we can all agree having an optic with a BDC calibrated to 900 yards makes more sense on a 7.62mm battle rifle than a 5.56mm carbine. If the BDC can't get me close with any commonly available 7.62mm load, then the optic will just be another "could have been" optic killed by it's own manufacture's lack of ability to see outside the box and actually create something useful, but then, I'm used to that by now. All the optics manufactures out there who want to claim ingenuity and innovation...I just don't know why a FFP optic with a math free rangefinder/BDC calibrated for any common 7.62x51 load (you know, a cartridge actually capable of being effective at ranges that require you to compensate for drop) with a low-end mag of 1.5 to 2.5x and a high-end mag of 6 to 8x simply doesn't exist on this marketplace, yet we can look through pages of overhyped mil and MOA based hash reticules. REALLY irks me how inventive and un-creative the optics industry really is...

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Z-Michigan
November 15, 2010, 08:51 AM
http://www.handloads.com/calc/index.html

M855 has a BC around .31. Muzzle velocity is your pick from about 3100 fps in an M16, 3000 in a 16" or maybe 2950 in a 14.5".

GunTech
November 15, 2010, 09:08 AM
Spec'd velocity for M855 is 3025 fps out of a 20 inch tube. BC for M855 is 0.301 G1, or around 0.183 G5. What range zero?

MTMilitiaman
November 15, 2010, 12:09 PM
The stated ballistics for the reticule are a 62 gr FMJ @ 2810 fps zeroed at 200 yards.

People have always told me that the 5.56's M855 ball round and the 7.62's M80 ball round share similar exterior ballistics out to any practical range. The exterior ballistics software that came with my bro's Nightforce gives a 62 gr FMJ with a BC of .31 and a muzzle velocity of 2810 fps ~365 inches of drop at 900 yards with a 200 yard zero. Winchester USA's 147 gr FMJ for the 7.62 NATO has a stated BC of .418 and an advertised muzzle velocity of 2800 fps. With those numbers, you get ~275 inches of drop @ 900 yards, with a 200 yard zero. That's not all that close IMO. Even running the 7.62mm load at 2700 fps, which I've never chronoed it but I assume that is more reasonable from a 22 inch barreled gas gun, you're still looking at just under ~300 inches of drop @ 900 yards, which all else being the same is still over five feet of difference between it and the 5.56mm ball load. So I just don't see this reticule being applicable to a rifle actually capable of reaching out to 900 yards. O well, another flashy optic with no application in the real world...

Here's a link to Leupold press release about the reticule, and an image of it:
http://www.opticstalk.com/new-leupold-cmr-reticle_topic25281.html

I've been dreaming of Trijicon putting their horseshoe-dot 7.62mm ACOG reticule on a 1.5-6x Accupoint with a 30mm tube for years. The new Leupold reticule is nearly perfect for what I want except some doofus decided in their infinite wisdom that they were going to make the perfect Designated Marksmen Rifle optic for an entry carbine instead of an actual Designated Marksmen Rifle. Hopefully they come out with a 7.62mm version on their 2.5-8 MR/T and thereby make that optic a legitimate DMR optic rather than just an underpowered sniper rifle optic, which is all that particular optic amounts to me currently available with Leupold Tactical Milling Reticule.

henschman
November 15, 2010, 01:58 PM
Yeah, I am with you guys... I wish Triji would make something with the ACOG reticle in a tube optic so it would be easier to put on an M1A.

For my M1A, I just went with the TA-55A ACOG. It is fixed 5.5x power and has a 7.62 BDC reticle out to 1200m. Mounted on an ARMS #18, it has about the same height over bore as on an AR. I had issues using it with a GI stock and a cheek rest though... I could get the cheek rest high enough for proper cheek weld/sight picture, but my shoulder still had to be down where the butt is. In other words, there is a lot of drop at heel, and it stretched my neck up too far for a comfortable prone position, and caused sideways pressure on the stock that kept me from getting in a relaxed position.

However, I think the Vltor Modstock I recently ordered will fix that issue, since the whole buttstock assembly is adjustable for height instead of just the cheek pad. Also, the collapsible stock will make it easier to get the proper eye relief for the ACOG.

I really love the scope though. It has the clearest glass I've ever seen. The BDC is also dead nuts out to 700 yards on man-sized silhouettes using surplus ball ammo, which is as far as I've been able to test it so far... and that's with the non-optimal cheek rest setup I described above.

As far as trajectory goes, the 5.56 and 7.62 are reasonably close out to 500 yards. The 5.56 is a little flatter-shooting at the closer ranges, and it fades away a LOT faster at the longer ranges. The standard come-ups for military 5.56 ball, in MOA, from 100 to 500 yards, are 2,2,3,4. The standard come-ups for 147 grain 7.62 are 3,3,4,4. Those are in 100 yard increments, starting at 100.

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