New Shiny Thing: Mk.262

At my club, 200 yards is the max I can reach out to, so the regular old 55 grain FMJ-BT is my bullet of choice. These days what powder I use depends on where I can get some for a very low price or free. Since I’m still going through powder I have scrounged from various sources, I’m not all that familiar with the wide array of choices. I generally have been sticking to Varget, H322, IMR4895 and I just came across a few pounds of Varmint and IMR4198.

I know a couple of guys who occasionally head to a 600-yard range near Atglen to do some long-range silhouette shooting. I haven’t really shot Silhouette in years, because it’s honestly about as much fun as watching paint dry if you’re not getting better. I don’t have the time, patience, or desire to master the sport. Those Silhouette guys helped improve my shooting a lot, but there came a point where it had done everything it was going to do for me without devoting more time to the game than I was willing to give.

But turn it into a precision rifle event with half-scale animals at ridiculous distances? You once again have my attention. That’ll be fun even if I’m missing a lot, and it gives me an excuse to tinker, which never gets old.

However, the most precisionist rifle I have is my AR, which isn’t anything to write home about. I bought it during the height of the ban almost 20 years ago by this point and it’s 100% stock, save the bayonet lug and birdcage flash suppressor I put on after the ban. It has regular old A2 furniture. A few weeks ago I uncased at the club and had some young guy say, “Wow. That’s old school!” It seems like just yesterday it was me saying shit like that to grey haired shooters. I resisted the urge to come back with, “You know, back in my day, we didn’t have any of these free-floatin’ barrels and fancy-dancy ACOGs with illuminated reticles.”

Old school or not, I’m thinking of slapping a halfway decent optic on it, and taking it out to see how I do at that distance with so-so equipment. But no way I’m taking my regular 55 grain load. I’m interested in the capabilities of the Mk. 262 round for reaching out to 600 yards. For those of you who may not be familiar, the Mk.262 is a 5.56x45mm round developed by SOCOM for longer range work. It uses a 77-grain Sierra Match King bullet. It is about as decent a round as you can get and still be able to stuff them into a magazine. I want to work with a practical round and not with loadings that have to be single-loaded into the rifle.

The trouble is, I’m not used to working with bullets that heavy in this chambering, and I’m paranoid about overpressure. A few months ago, I ran into a supply of 75gr HPBT bullets, and I am starting with those. That bullet with 22gr of H322 driving it isn’t yet a compressed load at 2.26″ OAL. Hodgdon says that’ll push out at about 2785ft/sec with 48,100 CUP from a 24″ test barrel. For a 77gr bullet, 21.8grs of H322 pushes 2,721ft/sec a 50,900 CUP. Black Hills seems to get close to 2900ft/sec out of a 20″ Colt AR barrel, so I’m guessing they have to be loading over the SAAMI spec and closer to the NATO spec on chamber pressure, which is about 10,000CUP higher. Anyone have any experience trying to replicate this load to get the same performance as the factory Black Hills load?

22 thoughts on “New Shiny Thing: Mk.262”

  1. I wouldn’t try to replicate Mk262 with handloads (~2750-2800fps out of an 18″ barrel, e.g. the Mk12 type rifles). That’s 5.56 kind of pressures, which is probably too hot for handloads (5.56 ammo is only meant to to be fired once in the military, after all). I’ve usually loaded them to around 2650fps or so. The 77gn bullet is common in highpower/XTC shooting, where people seem to mostly load to that velocity (somewhere around 24gn of Varget).

  2. Get some 8208 XBR, CFE223, and AR Comp and use Hornady’s 5.56 data. You can get some very good results with that. If you’re using Lake City brass just see what the brass tells you about pressure from ejector marks.

  3. That’s not old school.

    I take a Baker rifle to the range and shoot that at 200 yards. People shooting ARs and tacticool bolt actions can’t believe you can hit a silhouette at 200 yards with a flintlock and open sights.

    1. One of the most fun events I’ve ever shot is black powder silhouette at ranges from 25 to 150 yards. Each target scored based on size and you had 25 shots. Two hour time limit for your shots with four shooting positions. It was a “grocery match” where all the entry fees were pooled, the organizers went to the grocery store and bought a bunch of stuff. At the end of the match you went down the list in score order and each person chose an item. Usually enough stuff for 3 or 4 items per person. Shot well and you got steak. Otherwise, cheap soda and chips. Much fun had by all.

      But the fun part was the pace. You loaded the muzzleloader, waited for a position and then took your shot on the bear, groundhog or squirrel shaped target. Come off the line, reload and repeat. Any muzzleloader, original, replica or modern, irons or optics permitted. Very low key.

      Taught me a lot about shot control and making every one count since it was like T-Ball. Everyone got 25 shots but unlike T-Ball, no guarantee of a hit. Make each shot count somewhere. I own two modern muzzleloaders in the hopes of doing it again. It was that much fun.

      So I’m with you on this!

      1. I do primitive biathlons. It’s like a regular biathlon, but instead of skis you use wooden snowshoes, and instead of modern target rifles, you use muzzleloaders.

        It’s a timed event, and your score is your time. Generally you’ve got 9 shots, on a course that’s about a mile and a half long. Four shooting stations (2,2,2,3), and you’re shooting at steel plates. Every hit takes 5 minutes off your time.

  4. I don’t know jack about mk262, but in service rifle this seems like old hat. 80gr was too long to keep in the mag but as I recall the 77 grain hpbt MK (minus the cannelure) went in fine. Jam it full of RL15 and shoot it all day long at 300 yards. I did this out of an “old school” profile NM Armalite AR before flat tops were legal (or at least common at all).
    Highly recommend the book “Handloading for Competition” by Zediker. Very valuable book, especially for the system-minded. Even if you aren’t competing, the principles and such will still help.
    Guns and Ammo this last month had a good article on modern powders that I thought was interesting, including bits on single-base, double-base, deterrents, burn rate, temperature stability, and other geekery.
    But I’d really look to the service rifle / high power world and see what powders they’re using these days, as they shoot these rounds with a purpose. I’m (ignorantly) guessing the mk262 itself was developed based on the capabilities and requirements born out in the high power comps.

    1. By “legal” I meant in comp, not in law, if that wasn’t clear.

  5. Damn. All of this AR-15 and 5.56mm talk.

    My gun club is loving the M1 Carbine and the .30 Carbine Right now. 12 Guage Pump Action Shotguns are neck and neck with them.

  6. If you are slow firing at 600 yards, what’s the beef with single loading?


    Happy shooting!

  7. I’ve been shooting Hornady’s 75gr OTMs in 3Gun for a year now out to 500yrds. They cost half of Sierra’s bullets and have a better BC (.395 G1 advertised). I’ve had luck in the midrange of Nato pressures and I’ve been really happy with them.

    1. Twist rate would be my question too with that old of a rifle, it should in theory take a 1:8 to get the most out of 77 grain…

  8. Both my ARs are 1:9 twist, which I know isn’t ideal for a bullet that heavy. That’s why I’m starting with 75gr bullets I got for free.

    I’m thinking Joe’s idea of using 70gr might be a good one. Still pushing beyond ideal, but a 1:9 barrel has a better chance of stabilizing a 70gr bullet than a 77gr one.

    1. Berger has a twist rate calculate web app:

      The 70 grain bullet I suggested has a length of 0.976 inches. The rest of the information you need is here:

      The product information suggests a 1:9″ or faster twist but the twist rate calculator says that is “marginally stable” unless you are pushing it faster than I expect you can or you are shooting at a high elevation and/or high temperature.

      1. That’s handy, thanks! We’re basically at sea level here. Have you ever used this interior ballistics software? Not cost prohibitive, but expensive for something I don’t know if I’d use much. But I like that it can tell you things like how much powder you’re theoretically wastefully fireballing out the barrel. I’d probably definitely think about this if I were doing a lot of wildcatting.

  9. you might try some factory 69 grain loads to see how your rifle likes them. IMI is pretty good stuff and on sale lots of places right now. Quazi match grade for under 50 cents a round

  10. As a general rule I get best results with 75-77gr bullets in my 1:7 20” AR, while the ones with 1:9 are 16” and seem to do best with 68gr projectiles. I’ve not done much shooting over 300 yds or so. Just experiment. It’s the best teacher anyway!

Comments are closed.