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  1. #11
    Join Date
    Oct 2003
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    3,381
    Quote Originally Posted by ronlassit View Post
    Maybe they are turning the gas off and manually cycling the bolt to get a little more gas behind the bullet?
    If you're referring to the link(s) I posted, they weren't using a semi-auto, they were using a bolt gun. Also they ammo they used was listed.

  2. #12
    Join Date
    Sep 2009
    Posts
    8,664
    I just got my larue lower ordered. I will probably do a 16” or 18” obr rifle.... Idk which one yet, I won’t be using a can but I see no reason at all not to use a 16” barrel. It sounds to me from the mixed answers I get you go below 16” is where the FPS numbers really start to fall off probably due to powder not burning all the way.... Would be my guess but sounds to me that’s where the cut off needs to be for a long range rifle.
    Nothing says Fuck You like a shotgun.....

  3. #13
    Join Date
    Sep 2009
    Posts
    8,664
    16-18” just depends on the barrel length of my other 308 bolt gun. I want drop to be the same or close to the same....
    Nothing says Fuck You like a shotgun.....

  4. #14
    Join Date
    Jul 2014
    Posts
    408
    I would go with the 18" barrel if you plan on using your ar-10 in a sniper role. There is no substitute for speed, but everything is a compromise in ballistics. The fastest loads may not be anywhere near accurate enough to fit the bill. If you reload, that helps out a lot, as you can make bullets that work the way you want them to. In a .308 platform, TAC and AR Comp get better velocities than the usual recipes such as Varget, RL-15, 4064, 4895, but they might not be accurate in your rifle at the velocity you want. If you do reload, I would suggest getting a bullet comparator set and the Hornady concentricity tool if you do not already have them. They do make a difference, especially at distance. Hope all goes well in putting your rifle together. Let us know how it all turns out.

  5. #15
    Join Date
    Feb 2011
    Location
    Central Texas
    Posts
    1,391
    Quote Originally Posted by LawDog View Post
    The math is actually much more complicated than that. It's like the difference in dating ages. Three years isn't that much in your 20s, but it's a world apart between teenagers. You won't see much difference taking a 22" barrel back to 21", but you'll notice the difference between a 14" barrel and a 13" barrel. I wish there was a reliable formula that we could follow to accurately predict velocity loss across various barrel lengths, but thus far the only method of measuring this seems to involve chopping down a barrel to a nub of steel, and measuring velocity loss along the way. Add to that difficulty the fact that different bullet and powder combinations create very divergent results, so the data obtained would vary from one load to another.

    You can use 25 fps as a rule-of-thumb, but do keep in mind that it is a very, very rough estimation.

    Additionally, the effect of bullet velocity on terminal performance is subject to this same criticism. The difference between 2800 fps and 2775 fps isn't as sharp and noticeable as the gap between 2200 fps and 2175 fps. The biggest hurdle is the speed of sound. Everyone agrees that bullets go all wonky as they transition through the transsonic zone, but there isn't a consensus on exactly where the transsonic zone begins (at least in regard to bullet performance). We can pinpoint the exact speed of sound, but there is a buffer zone somewhere above that point where bullets begin to lose their stability. Looking at my own DOPE, I believe that point begins somewhere around 1200 to 1300 fps. I can't actually measure my bullet velocity at range, so I'm trusting a ballistic calculator for that data point. But my long range shooting falls apart around the point that the bullet drops into that 1200 to 1300 fps zone. The upside, when we are talking about .308 Win, is that we aren't as dependent on velocity for terminal performance. With a 5.56, there is also a great debate on how slow a bullet can move and still demonstrate sufficient terminal effect on the target. That's a factor here, but it isn't nearly as important.

    Now I'm going to spend the rest of the day wondering how much money I could make as a Youtuber. I would love to get paid to conduct scientifically valid experiments on guns--chopping them down and measuring their data points along the way. But I would definitely need an ammo sponsor for that.
    This is a thread drift, but back in the Mid 70s’ I was a tank gunner. We fired the M68 105 MM Main Gun at ranges up to 3800 M, but usually between 1200 to 2800 M. The HEAT round would go trans sonic between 2500 and 2600M, at which point 50% of the time the round would go up, down, right or left with no predictability, and rest of the time continue straight in for a solid hit. Humidity’ temp, and barametric pressure all played a part. The HEP and SABOT rounds both stayed supersonic out 4800 M and beyond so they did not suffer from this problem.

    jim
    2 Samuel 22; Psalm 139:21-22

  6. #16
    Join Date
    Jan 2012
    Location
    NW Washington
    Posts
    3,034
    Quote Originally Posted by Chainsaw76 View Post
    This is a thread drift, but back in the Mid 70s’ I was a tank gunner. We fired the M68 105 MM Main Gun at ranges up to 3800 M, but usually between 1200 to 2800 M. The HEAT round would go trans sonic between 2500 and 2600M, at which point 50% of the time the round would go up, down, right or left with no predictability, and rest of the time continue straight in for a solid hit. Humidity’ temp, and barametric pressure all played a part. The HEP and SABOT rounds both stayed supersonic out 4800 M and beyond so they did not suffer from this problem.

    jim
    Neat!

    Same reason accuracy from 168gr 308 starts to fall apart sooner than the 175gr loads, and why the 6.5 bests either one easily.

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