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What TWIST RATE for which bullet weight |
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Stevey
Optics GrassHopper Joined: June/24/2010 Status: Offline Points: 39 |
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Yes, boat tail bullets can shoot sub MOA @ 100 yds but bench rest matches require better than that. A 70 gr 6mm PPC can kill small bugs at 100 yds. A 45-70, 300-500gr. can make it out to 1000yds but at that range it has been sub sonic for almost half its flight (blooby trajectory). Help me out on this ... is Hookes law, as pertaining to the elastic qualities of metals ? (bullets in snow banks) causing the bullets to contract after being obdurated by either expanding gasses (rapid pressure increase) or differences in acceleration forces applied to the end of the bullet as opposed by the other pointy end that is at rest at the start of acceleration from say 0 to over 3000 fps. Is a stubby mini ball with its hollow base puffed up by a blast of black powder? Has anybody ever seen signs of gas blow by from firing a tightly fitting jacketed rifle bullet through a precisely matched rifle bore? My guess is they either slide through cleanly or get stuck -- possibly there is no room for obduration.
If in doubt go for a faster twist.
My feeling is that B.C.'s vary with velocity as much as 10% (or possibly more) due to primarily coneing and any effect due to obduration is zippo as acceleration stops after the bullet is launched and providing the elastic properties of the bullet are not exceeded the bullet assumes it's prelaunch diameter except for rifling and other marks.
I have 2, 1-8 twist .243's that shoot 107 & 105 gr boat tail bullets just fine -- even up to 1000yds but I have not been able to hit a small bug at 100 yds with either of them. I would guess a 1-8 twist would be marginal with 115, 6mm's and 1-7 would be better -- the observation of keyhole's at 100 yds and not at 50 yds is of note, I guess things just keep on going bad down range.
I have noticed the groups fired with 142 gr SiMK (1-8 twist) at 100 yds are only moderately better than those fired at 300yds. Is there any truth to the statement that long pointy bullets "go to sleep" at some extended range.
Are rebated boat tail bullets still sold? My guess is that there is no advantage to them as obduration is not a factor
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Stevey
Optics GrassHopper Joined: June/24/2010 Status: Offline Points: 39 |
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On the obduration question -- at say 50,000 to 65,000 psi anything that can be deformed or squashed will be obdurated and accelerated (progressive gun powders). Acceleration is a result of a force rather than causing a force.
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helo18
Optics Jedi Knight Joined: December/02/2006 Location: Montana Status: Offline Points: 5620 |
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Thanks for the bump Wes. I was looking for that chart recently.
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budperm
Optics Retard show me your sheep!! Joined: January/01/2009 Location: Pennsylvania Status: Offline Points: 31710 |
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Can a moderator Please Pin this post!
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"Peace is that brief glorious moment in history when everybody stands around reloading".
--Thomas Jefferson |
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Bigdaddy0381
MODERATOR Georgia peach Joined: February/27/2007 Location: Georgia Status: Offline Points: 13682 |
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this is the thread I was looking for.
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P&Z Firearms , Pro gun cleanings and gun repair and wood refinishing.
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budperm
Optics Retard show me your sheep!! Joined: January/01/2009 Location: Pennsylvania Status: Offline Points: 31710 |
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Yepper, its definitely one of the better ones!!!
Good help to us all not just newbies!
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"Peace is that brief glorious moment in history when everybody stands around reloading".
--Thomas Jefferson |
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xiaochun3612
Optics GrassHopper Joined: May/31/2011 Status: Offline Points: 3 |
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I would try both to see which provided better accuracy, if they are
equal then get teh 175. It will fight the wind better than the lighter
grain. With a 10 twist you may even be able to put some bullets up to
200gr or even the 208 hornady a max. I know this doesn't really answer.
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Believe myself.
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snakeman48
Optics Apprentice Joined: March/29/2010 Location: SW Lower Mich Status: Offline Points: 129 |
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Finally found this chart, again Been lookin' for it, found it, then lost it. Thanks |
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NRA Benefactor Member. Veteran; US Army 5th Corps 1970-1972
The only time in my life when I got to Rock-N-Roll, and the ammo was free!! |
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River Runner
Optics Apprentice Joined: January/24/2012 Location: Colorado Status: Offline Points: 50 |
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So if I know the twist rate of the barrel on my gun, should I choose a bullet weight based on that twist? And is the twist rate easy to find out? I have a Winchester model 88 in .308. |
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Randy
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3_tens
Optics Jedi Master Joined: January/08/2007 Location: Oklahoma Status: Offline Points: 7853 |
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Each barrel has different dynamics. Even 2 barrels that are made by the same machine one right after the other will shoot different. Learning your weapon comes from careful notes maintained over time. Then about the time you get it all figured out its time to re-barrel the rifle and start all over again.
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Folks ain't got a sense of humor no more. They don't laugh they just get sore.
Need to follow the rules. Just hard to determine which set of rules to follow Now the rules have changed again. |
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Stevey Ducks
Optics Apprentice Joined: December/03/2011 Status: Offline Points: 266 |
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1 - 11 twist for .308 diameter bullets 190 gr or more?
1-14 gr. for the real fast stubby ones.
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338LAPUASLAP
Optics Master Scope Swapper Joined: October/17/2009 Status: Offline Points: 2596 |
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GOOD Thread BUMP even though it is a sticky icky...
I have found limits as far as speed and weight that are factors as well as barrel material and heat of barrel and specific heat properties a factor as well as bullet material hardness and coef rise. BUMPED |
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No one
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nralifer
Optics Apprentice Joined: July/26/2016 Status: Offline Points: 165 |
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I thought I might post something on this thread since there has not been one in a long time. The subject of stability and barrel twist rate is something that I know a little about since I have had to deal with it a lot. Stability is a function of bullet length, caliber muzzle velocity, air density and all the factors that influence that, and bullet weight. The simplest thing to do is to go to the JBM Ballistics site and play with their bullet stability calculator. To be sure that the bullet one wishes to try will be stabilized by a barrel of a given twist simply measure with a calipers the length of the bullet, weigh it, and plug in the numbers into the JBM calculator. For atmospheric variables one can use standard conditions such as 59 degrees F, 29.92 in Hg, and 50% humidity. For a bullet to be stable under most conditions, a stability factor of at least 1.4, and preferably 1.5 is needed. As the atmospheric pressure increases and the temperature decreases, the air gets denser and the stability factor decreases. A bullet, when shot in the summer, may appear stable, but come winter time the stability may not be there (hit sideways or shoot bad groups on cold days). When the stability factor under a given set of conditions falls below 1.25 then weird things can happen like it is accurate at 200 yds but key holes at longer ranges. Also it will have a tendency to tumble through the target and have erratic penetration through an animal since flesh is denser than air.
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It is hard to fail, but it is worse never to have tried to succeed. TR
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Peddler
Optics God Joined: July/04/2012 Location: Oswego,NY Status: Offline Points: 13533 |
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Welcome to the OT George.
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When you are dead, you don't know you are dead.It is difficult only for others.
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Bigdaddy0381
MODERATOR Georgia peach Joined: February/27/2007 Location: Georgia Status: Offline Points: 13682 |
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This is my best load to date. I like to use the side of the bullet for a better energy dump. The bullet dose not mushroom out like some I have seen, but they do have good knock down power. To get this fine groups at 200 yards I have to use a slower twist and heaver bullets.s This is 200 yard 5 shot group. |
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Ecclesiastes 10:2 |
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nralifer
Optics Apprentice Joined: July/26/2016 Status: Offline Points: 165 |
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Thank you Peddler.It's a really interesting forum. Enjoyed Bigdaddy's pic. Looks familiar. Have a great friend who likes to machine his own bullets for this monster 338 Lapua. He made a 325 gr machined copper bullet about as long as the Texas panhandle that actually shot sub minute of angle groups out to 3-400 yds. (not sure which one now). We wanted to test it at a longer range and so we set up a target at 500 yds. It was a a very cold day (5degrees,and pressure was 30.30 in Hg). Shot three shots, and all the holes looked like Bigdaddy's pic. The funny thing was the group was minute of angle! We actually spoke to a Hornady ballistician about the phenomenon and he said it was not possible. Glad to see that someone else has witnessed the impossible as well.
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It is hard to fail, but it is worse never to have tried to succeed. TR
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DCAMM94
Optics Master Extraordinaire Joined: April/19/2008 Location: Fort Worth Status: Offline Points: 3491 |
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I once had groupings like that (consistently) out of my old .243 Encore 15" handgun. The only thing it would shoot consistently was 85gr, and I had to use compressed loads or the older hornady overcooked loads. It was a 1-11" I believe. Now I use a 7-08 barrel with a 1-9 twist, and it will hold a spiral a little better than a Billy Kilmer pass - at least out to 300yds - with up to 140grainers.
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Although personally I am quite content with existing explosives, I feel we must not stand in the path of improvement. -Winston Churchill
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nralifer
Optics Apprentice Joined: July/26/2016 Status: Offline Points: 165 |
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Since that experience if there is a question about stability we try and test in cold weather and for 500 yds or better.
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It is hard to fail, but it is worse never to have tried to succeed. TR
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