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AR bullet weight?

RonboRonbo Member Posts: 39 ✭✭
edited June 2015 in Ask the Experts
Hi folks, I am looking to get some heavier ammo for longer range shooting but don't know if my current rifle can handle it? I was looking at the Black Hills 77gr ballistic tip.... is that too heavy for a 1/9 ? I'm thinking I would have to move to a 1/7 or at least a 1/8. Yes? No?? Maybe???
Also, will this size round work in a pmag? Reason I ask is because I read somewhere a while back that Hornady was the only one to make a round this heavy that would work in one due to how they seated the bullet. I don't know if this is true or not as I have only used 62gr up to this point.
Any and all input will be greatly appreciated!
TIA [8D]


  • CapnMidnightCapnMidnight Member Posts: 8,520
    edited November -1
    You'll need a faster twist than you 1/9". I would say 1/7". The 1/8" may not be enough to stabilize the 77gr bullet. There will be folks along shortly with more knowledge on the AR than I have.
  • tsr1965tsr1965 Member Posts: 8,682 ✭✭
    edited November -1
    The Capn, is right on the money. To be sure, go to the 1 in 7. The 1 in 9 is definitely not, and the 1 in 8 is iffy, though it will stabilize the Barnes 62grain TTSX, and the 70 grain TSX.

    Also, just so you know, that is not a 77 grain ballistic tip, by Black Hills. It is a 77 grain Sierra Match King. I just looked, and there is a god chance a 1 in 8 will do it, but since you have to switch from a 1 in 9 anyway, I would go to the 1 in 9.
  • MobuckMobuck Member Posts: 13,050 ✭✭✭✭
    edited November -1
    I suggest you try a 69 HPBT before attempting (or wasting money)on 77 grain bullets.
  • 62fuelie62fuelie Member Posts: 1,068 ✭✭✭
    edited November -1
    I have to agree with the Capt. I have a 1 in 9 on my AR and the 69grain Sierra runs just fine in it. One of the problems you run into with the heavier bullets is that they take a good deal of powder room. You would get better performance with a 22-250 with a quick twist barrel and the heavy bullet. The monolithics ( all copper) tend to be longer that the cup and core bullets so they present even more problems stabilizing them and driving them fast enough to be useful at long range.
  • dcs shootersdcs shooters Member Posts: 10,969
    edited November -1
    My 1/8 shoots the 77gr,s just as good as a 69gr [;)][^]
  • Ray BRay B Member Posts: 11,822
    edited November -1
    According to Sierra, the 77 gr TMK requires a 1-8" or faster twist. since they make the bullets I'd put faith in what they say.
  • sandwarriorsandwarrior Member Posts: 5,453 ✭✭✭
    edited November -1

    1-9" will stabilize that bullet if you are getting full velocity. Meaning a rifle length gas path on a rifle length barrel. Where it may be affected is when running these bullet weights (lengths) out of a carbine where you lose velocity. Stability is on the edge with these and 200-300 fps lower velocities. I shoot these all the time out of my full length AR-15.
    That said, a 1-8" will stabilize them better. And, through the transonic range. So, if you want to reach out past 1k with them, (yes, an AR) go with a 1-8"

    There is no need to find a 1-7". The 1-7" was put on our M4's so it could stabilize the extremely long tracer bullet. That's why the initial -A2 went to 1-7" from 1-9" It was first tendered in a 1-10" twist. Which stabilized the 62's. They then "one-bettered" it. Then some genius thought we ought to have a tracer that reached as far as the 62 gr. bullet. That takes a lot of burn marker filling up the back of an otherwise almost hollow bullet.

    A 1-7" is useless on an AR because anything you can run through a magazine will stabilize with a 1-8"....except tracer. Again, a waste, they are a marker bullet not a precision bullet.

    On the magazines. P-Mags are about the shortest length mags I've used. So running heavier bullets out long is not possible. My Gen2/3 mags are at 2.263" inside. So, that gives you .003" over max length. Not much. You can still work up full power loads though. My 75/77 gr. load is at 2980.
  • nononsensenononsense Member Posts: 10,933 ✭✭✭✭
    edited November -1

    Lots of opinions and little hard information.

    Right off the bat, The 77 gr. Sierra Match King will stabilize in a 16" barrel as long as you are above 2550 FPS. Remember though that this is only good ABOVE that velocity. The bullet is 1.000" long and is seated 1/2 way into the case (0.500") using up 4.75 grains of powder room. My load using TAC and the MK yields 2650 FPS from my carbine at 16 inches. It will also make it to 1,000 yards but it is trans-sonic and potentially unstable. I use a 24" barrel and shoot another load single shot with the 90 gr. bullet seated out to gain what I need at 1,000 yards.

    So, yes you can use the 1:9" twist but it operates in a more narrow velocity band.

    To be more than adequately safe for a stable bullet, the 1:8" twist will be suitable. However, the most available barrels are the 1:7" twist and this will be just fine because you will never achieve enough velocity to strip the bullet in that faster twist rate. And if you ever decide to shoot the really heavy bullets single shot, you'll have the twist available. Your chamber may not be long enough but that's why we use throating reamers...

    Last, while the 69 gr. MK is a superb bullet in it's own right, the reality is that it drops and drifts and additional 20" when compared to the 77 gr. MK when shooting at 1,000 yards. Yes, you're already dropping a significant distance but this is a situation where more is definitely NOT better.

    Good Luck with your decision and please come back to tell us about your experiences.

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