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Newbie AR Q&A

Discussion in 'General Firearm Discussion' started by OEDub, Dec 4, 2009.

  1. OEDub

    OEDub SW OR Coast Active Member

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    I'm trying to gain info from people who really know their AR's before I make the investment to purchase my own. I shoot left handed & have been looking into Stag.

    My first question is: what is the pros vs cons of a 1:9 & 1:7 twist barrel and what do you prefer/reccomend?

    Second: pros vs cons of a piston system & preferences/reccomendations?
     
  2. UncalledForGabe

    UncalledForGabe Aloha Or. Member

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    1:7 = heavy match ammo (77gr.)
    1:9 = standard bullet weight (55gr.)

    Im not sure weather or not its good to shoot light weight bullets through a fast twist barrel. Like a 55gr. through a 1:7. Might be a little harsh on the rifling. 77gr. and similar will not stabilize at longer range in a 1:9 barrel.
    Good questions!
     
  3. the4thshake

    the4thshake Portland Active Member

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    I have shot 45 grain bullets out of a 1-8 twist and they made it out to 100 yards without spinning apart. With a 1-7 twist the lowest I have gone is a 55 grain. I like the faster twists because I shoot heavier bullets. The 1-9 barrels may or may not shoot the heavier bullets well.
     
  4. tonyspdx

    tonyspdx Gresham, OR Member

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    I agree, 1:9 has good performance with 55gr (best) and 62gr.
     
  5. Gunner3456

    Gunner3456 Salem Well-Known Member

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    The cheapest and most common ammo is 55gr and shoots best in 1:9. Unless you reload or have specific plans for heavy bullets...

    Military spec .556 ball ammo most commonly available at decent prices (often M193) is 55gr.
     
  6. spengo

    spengo GLORIOUS CASCADIA Active Member

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    1:7 twist will work good for anything between 55gr and 77gr bullet weights, 1:9 will work good for anything up to 62gr but I dunno if I'd try running heavier bullets in it than that.

    Personally I think the 1:9 twist is a bit silly, it's a compromise twist rate. If you want the gun for varminting, using tiny 40gr bullets get one in 1:12. Anything else, go with 1:7. Both 1:7 and 1:9 will be fine for 55gr, but if you want to shoot 75-77gr defensive or match loads I think you'll find they keyhole out of the 1:9 barrels.

    Here's some proof that cheap bubblegumty 55gr ammo works just fine out of a 1:7 barrel.
     
  7. Gunner3456

    Gunner3456 Salem Well-Known Member

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    I always hate to be negative, but that's not a very good group for an AR-15.
     
  8. the4thshake

    the4thshake Portland Active Member

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    I have a 1-7 twist that will shoot a group of 5 with XM193 under an inch at 100 yards. Another 1-7 twist I have, with the same ammo, shoots groups double that size if I'm lucky.
     
  9. wakejoe

    wakejoe Beaverton, OR Well-Known Member

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    Chrome lined 1:7 twist. You can still shoot light weight bullets perfectly fine, no reason to limit your ability on heavier (More accurate) ammunition.

    No piston, stick with DI. Carrier Tilt is bad news.

    Full auto carrier, not semi auto. Make sure it's MP tested.
     
  10. whphel

    whphel Lake Stevens Member

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    That is iron sites standing I hope! Oops, I just noticed the writing target. Thats prone, sorry but that is not a very tight goup for 100 prone with a .223.
     
  11. Gunner3456

    Gunner3456 Salem Well-Known Member

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    I don't really want to argue but the OP asked a serious question and is going to make a serious investment.

    First, it is not the weight of the bullet which matters for twist rate. It is the length of the bullet. A long bullet must spin faster to stabilize. It just happens that if bullets have a similar design (similar cross-sectional density) such as copper jacketed lead, the heavier bullet will be longer.

    Tracer bullets are, for instance, by design lighter in cross sectional density but much longer than lead core bullets. Lighter here requires a faster twist rate because it's longer.

    Get this. The first AR-15's put into military service in 1960 (in 'Nam) had a twist rate of 1:14. The result was devastating on-target with a 55 gr bullet.

    The first M-16's had twist rates of 1:14 and soon after 1:12, again shooting 55 gr bullets.

    The only reason the military went to some 1:7 rate barrels was so that they could opt to shoot tracers! Period! The lighter and heavier (55 - 62 gr) ball ammo worked OK in those barrels but it was a compromise.

    If you really want to drive nails with an AR-15 using 55 - 70 gr FMJ ball ammo, get a 1:9 twist, or better yet stick to 55 - 62 gr and a 1:9 twist.

    There is also a good reason that most AR-15 manufacturers ship a 1:9 twist as their default barrel. The reason? They know what they are doing.
     
  12. spengo

    spengo GLORIOUS CASCADIA Active Member

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    :( I suck then. Yes iron sights, I can't afford optics atm. :/
     
  13. Spray-n-pray

    Spray-n-pray Battle Ground Moderator Staff Member

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    Dude! I don't want to pee in anybody's cheerios, but that group is just fine. You were using iron sights, with lousy brown bear (not the most accurate), and all were within about six inches. If you were shooting matches, you would be using an optic of some kind and good ammo, and your group would probably shrink. For just punching holes in paper that is not bad. Don't forget, the AR is designed to be a battle rifle, and any one of those shots would have hit a person if you aimed center mass.
     
  14. spengo

    spengo GLORIOUS CASCADIA Active Member

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    Well I do get better groups when I use nice hornandy 75-77gr stuff, usually that's too expensive for me though. The stuff I buy normally is whatever the cheapest 55gr brass cased stuff is on the shelf at the time. If I use sandbags I can get it in a 2-3" group easy as pie.

    EDIT: hey hang on a bit... this is a 3" group too isn't it? You guys must practice a lot if you can do better than that with irons and brown bear ammo...
     
  15. tionico

    tionico Thurston County Well-Known Member

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    Hey, three inch at a hundred? In any combat situation, that means dead guys. And with dirtbag ammo into the bargain?

    Interesting details about barrel twist and how it affects the rounds... and different types of them.
     
  16. Murphy

    Murphy Oregon Member

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    For those of you who are complaining about the group in the photo could you please post some photo's of your latest groups?
     
  17. Gunner3456

    Gunner3456 Salem Well-Known Member

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    It's not just your group that's an issue. It's the mistaken theory behind it. It's not realistic to call a 1:9 twist "silly" when the engineers for virtually every AR-15 maker choose it and ship it as the default barrel for the guns.

    You will not see good accuracy with lead core bullets of less than 75 gr in a 1:7 twist barrel because the bullets aren't long enough to work with that fast twist!

    If you want the best accuracy you can get with 55 gr lead core bullets, get a 1:12! !!

    If you are shooting armor piercing, it will be steel, have a lower cross sectional density, and need to be longer than a lead core bullet for both to have the same weight. The steel bullet will need a faster twist for best accuracy only because it's longer!!

    Of course you do better in the 1:7 twist with 75 - 77 gr bullets. They are longer! That's it. Longer!

    People can blame ammo brands (might be some truth in that) or whatever, but as long as someone insists on shooting short (55 - 62 gr lead core) bullets in a 1:7 twist barrel, it's not going to group as well as it would in a 1:12 twist. Period.

    The 1:9 twist is the best compromise all around twist. The 1:7 is a specialty barrel not appropriate for most uses. It is a tracer round barrel or an 80+ gr barrel because they are longer!
     
  18. Gunner3456

    Gunner3456 Salem Well-Known Member

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    [Continue rant]

    The .223/.556 was never designed to push an 80 gr bullet. It's power and effectiveness is based on speed, not weight. It needs a muzzle velocity of more than 3100 fps to be effective. You can get that from 55 - 62 gr bullets, but an 80 gr bullet will wimp out in that area.

    If you really need to shoot an 80 gr bullet, you need an .243 or a 6.5 to get the speed, the best cross sectional density, and the best ballistic coefficient. If the task requires a heavier bullet, the .556 is simply the wrong platform.

    I believe that you'd risk blowing up a .556 AR-15 trying to push an 80 gr projectile to 3150 fps (the speed of XM193.)

    The AR-15 in .556 gets it's killing power from speed, not weight, and it's deadly. Even the 55 gr FMJ XM193 bullet usually tumbles and breaks up upon hitting an "animal" and the resulting wound channel is usually fatal.

    [/rant]
     
  19. spengo

    spengo GLORIOUS CASCADIA Active Member

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    Well I guess what I was trying to say is, 55gr groups acceptably. If you attach the rifle to a bench rest it should be ~1.5-2moa with a 1:7 barrel and 55gr bullets. Anything less will be very overstabilized though. I just wanted to show with my target that you aren't going to overstabilize the 55gr much if at all. You're right that 1:9 isn't silly but if you try to shoot 75gr+ stuff in it they will keyhole. The coolest home defense and target ammunition is 75gr and 77gr so that's why I chose it. If I recall, all 5.56x45 bullets use fragmentation as the main wounding mechanism, including the high grain boat tail hollowpoints. There are some expanding .223 JHP and JSP though.

    That's a very interesting thing about the bullet length, I haven't actually heard people talk about that before. Usually when people describe what type of ammunition works well they only talk about bullet weights and give a range like "55gr-80gr." When I think about it, length makes a lot of sense though. Also, is there any 55gr steel core or is that stuff all 62gr m855?