Barrel twist rate's affect on pressure

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by airstream, Mar 2, 2011.

  1. airstream

    airstream Well-Known Member

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    First off , I know verry little about pressure. I am currently waiting on a 260 rem from Cooper . The rifle is offered in 1 in 9 twist rate, it would cost me 440$ to change the twist rate to 1 in 8 to stabilize a bigger bullet better. I will be reloading 140 grain lapua's and Berger's starting out. Cooper says that they reach the accuracy gaurentee with 142 gr sierra's just fine. So im going with the 1 in 9.

    I am assuming that a 1 in 8 would give higher pressures?????

    When i start working my way up to a max load what is usually the first signs of pressure?

    the gun will have a 26 inch barrel. The longer the barrrel the higher the pressure?

    I have never had pressure issues as I always have just realoaded for the 243 out of the mannaul.
     
    Last edited: Mar 2, 2011
  2. backwoods83

    backwoods83 Well-Known Member

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    Good news is a 9 twist will stabilize those bullets at around 3200fps, bad news is if you want that velocity with a 260 case you may as well s(@t in one hand, wish in the other and see which one fills up quicker. From all I have seen twist and barrel length have virtually no effect on pressure. Depending on your brass ( el cheapo brass first sign of pressure most likely flattened primers leading to loose primer pockets) good brass (Lapua) your first sign will likely be a sticky bolt when lifting. Regroup 8 twist 6.5 creed or 6.5x284 good luck.
     

  3. Mikecr

    Mikecr Well-Known Member

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    Why are you trying to relate twist with pressure here?
    Do you think you'll have pressure problems?

    You just load a Cooper like any other gun. Their barrels aren't different or unique..
     
  4. J E Custom

    J E Custom Well-Known Member

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    Pressure comes from many things, but the twist rate is not considered one of them.

    The number of groves,chamber,free bore,bullet size and shape, Etc. all have more effect on
    pressure than the twist rate.

    Velocity is more effected by twist and grove count and the only twist rate that I know of the
    makes any real difference is a gain twist.

    Longer barrels can/may reduce pressure because of the use of slower burning powder,
    reducing peak pressure and increasing velocity.

    With all things being equal bullet seating depth (Jump it the lands)probably has the most
    effect on pressure.

    So dont make it difficult on yourself,just load according to the loading manuals and watch
    the primers and/or bolt lift.

    Have fun with the new rifle.

    J E CUSTOM
     
  5. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Airstream,

    A little something to ponder here. Military ammo is loaded hot, usually right up to the max pressures allowable. The original M16 rifles had 1x14" twists for use with the 55 grain FMJ M193 (3,250 fps from a 20" barrel). This twist was increased in the M16A1 rifles to 1x12", with no problems at all in the ammunition/pressure department. They've now gone to a 1x7" for our M16A2s, A4s and M4 platforms. They still produce and issue the M193 ball ammo for training purposes, again with no problems at all. Doubled the twist from 1x14" to 1x7" and it doesn't create a problem. No, that 1" difference won't mean a thing to you in terms of pressure.

    The other question I'd have here would be is the barrel buttoned or cut? Cut barrels are usually closer to their advertised twist rates, where buttoned barrels can vary quite a bit. Some makers are better about this than others, but I've seen them vary by well over +/- a half inch (one inch total) from their stated twist. With the 140-142 grain bullets, that's a bit too close to the edge, in my book.
     
  6. airstream

    airstream Well-Known Member

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    Ok, I got the picture. Now Kevin , yes the barrel is arriving with a ''target crown'' is this what you call a button? and if so please explain were your going with that. Thank you all for the comments.
     
  7. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Barrels from aftermarket makers are generally divided into one of two type; cut or buttoned. It refers to how the barrel is made. In a nutshell, a barrel is drilled to approximately bore size, the same in either method. In a buttoned barrel, the lands and grooves are formed by pulling (or pushing, in some cases) a solid carbide "button" through the bore. This button is oversize (as in, won't fit through the bore), and is a mirror reversed image of the rifling. As it's pulled through the bore, it literally swages the rifling into the bore surface, leaving a fully rifled barrel as the button emerges from the far end. One pass, very simple, very quick. In a cut rifled barrel, a single point (generally speaking) cutting head is pulled through the barrel, leaving a very slight scratch in the surface of the bore. Ever heard the term, "starting from scratch"? That's where it comes from; barrel making. Anyway, once the cutter emerges, the barrel is rotated 90 degrees (for a four groove barrel) and the cutter makes another pass, leaving another scratch parallel to the first. This is repeated two more times until there's four identical scratches down the bore. At that point, the cutter head is raised, and it goes down the first scratch for a second time, cutting it deeper. Bear in mind, we're talking millionths of an inch being removed at each pass, not hogging out a full groove in a single pass. The process is repeated in each of the succesive grooves, the cutter reaised again, repeat, etc.. Normally, that cutter will make somewhere between three and four undred passes up and down that bore before the grooves are fully cut. This of course depends on the number of grooves (I used four here, merely as an example), their depth, bore diameter, etc.. You're normally looking at 45 minutes to an hour or so for the production of a single cut rifled barrel once it arrives at the cutting process, where a buttoned barrel takes around ten to 40 seconds, depending on the manufacturer. Cut barrels (at least the better ones) will normally be lapped once completed, where very few buttoned barrels are. Both processes can produce good barrels, but there's advantages and disadvantages to each as well. Take your pick.

    The third common method of barrel making is hammer forging, but the machinery and tooling for this is well into the millions, so it's primarily a process for major manufacturers only. Ruger does some hammer forging of their barrels, Winchester did at one time, and I believe Remington is using it for some of theirs as well. Very quick, can turn out a good barrel (I've used dozens of Sauer 202 barrels that were excellent) and they're fairly cheap to produce, once the equipement costs are amortized.

    There are, or have been some others, but you don't see them being widely used today. It's an intersting history, and a fascinating process whichever method is used.
     
  8. Boss Hoss

    Boss Hoss Well-Known Member

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    Kevin---you might want to address the "upset" of the bullet when it impacts. Of course twist has a lot to do with this. This is something that Rich and I have discussed on more than one occasion.
     
  9. Kevin Thomas

    Kevin Thomas Well-Known Member

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    He had a simple twist rate question. Let's keep it simple for now. Poor guy's trying to make sense of this mess and you want to throw him a curveball?
     
  10. airstream

    airstream Well-Known Member

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    ''Start from sctatch'' ''bite the bullet'' ''fire at will''


    Kevin you have done an awsome job explaining things, thank you verry much for taking the time. I cant wait to tell my oddball friend about the origin of the phrase, we love wierd facts ( we are not geeks just wierd ).

    Any how I read about ''hand lapped'' barrels but have no idea what that means ?