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Old May 18, 2009, 08:24 PM   #1
WeedWacker
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What do higher twist rates do to lighter bullets?

More specifically, how would my 1:7 16" carbine upper do with a 45 gr .223 JHP? I've heard everything from keyhole to exploding barrel but I have no idea with my meager understanding of physics and explosive induced high velocity projectiles.
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Old May 18, 2009, 08:37 PM   #2
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A certain size bullet shot at a certain speed (fps) needs a certain twist rate to stabilize, ie shoot as accurately as possible.

When that doesn't match up, usually your pattern is going to be bigger. Sometimes it means your bullets will tumble (and keyhole the target) if it's super far off. I've never heard of a barrel exploding... sounds quite ridiculous, actually. Maybe someone else will correct me, though.

Someone please correct me if I'm wrong, but a reloader once told me that any weight bullet can be shot accurately with any rate of twist.. you just have to adjust the speed accordingly. For example, my riflle likes 168gr Federal GMM MatchKings (which shoot at 2700fps, I think). If I were to buy 135grain MatchKings , I could get the same accuracy once I found that certain speed at which the bullet will stabilize with my rate of twist.

Hope that helps.
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Old May 18, 2009, 09:05 PM   #3
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Well the really heavy .223 bullets need a faster spin rate to completely stabilize but in some some types of bullets, at high speeds, the centripetal force generated by a 1:7 twist will rip a lighter bullet apart. I'm not sure the exact details but it can happen. Try the 45 grain JHP and see, I think with a 16" barrel you may be OK but don't be suprised if your bullets don't make it to the target.
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Old May 18, 2009, 10:11 PM   #4
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Most of those little bullets are meant for varmints and are kind of fragile.
You'll probably get "Blue Streak" from them. That's what it looks like as they disintegrate in the air.
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Old May 18, 2009, 10:32 PM   #5
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It's the twist that makes the bullet spin so it's stable in flight.

The LONGER the bullet, the more twist is required to put enough spin for stabilization. Increased velocity will increase the spin, and, TO SOME EXTENT, make up for less twist.

Weight of the bullet is a factor because the heavier the bullet, the longer it is--but it's the LENGTH that really determines how much twist you need. For example, I have a custom 30-'06, with a 1 in 11" twist that will stabilize a 200 gr. rn bullet, but not a spitzer. It may stabilize a 220 rn, but I haven't tried it yet. Standard twist is 1 in 10". My rifle builder put a Krieger 1 in 11" twist in order not to "overstabilize" most bullet weights. For best accuracy, neither too much, nor too little twist is desired.

You see the importance of bullet length when shooting Barnes bullets. They're made of all copper, and therefore longer in relation to their diameter than lead bullets of the same weight. That's why some rifles may stabilize a 180 gr. Spitzer, but not a 180 gr. Barnes--it's too long.
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Old May 18, 2009, 10:52 PM   #6
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So, if I fire a 45 gr Varmint JHP Winchester factory load at 3600 fps in a 1:7" twist barrel - most likely it will disintegrate?
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Old May 19, 2009, 09:05 AM   #7
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I have yet to see a bullet vaporize, but you won't hit the broad side of a barn with that combo, mine wouldn't shoot 55's let alone what you have, try it anyway, it wont hurt anything, then try some heavies
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Old May 19, 2009, 11:06 AM   #8
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hmmm... the only problems with the lighter bullets in faster twist rates...

any imperfections are amplified by the faster twist rates ... ummm maybe a very fragile varmint bullet may come apart, but it's highly unlikely ( they make the same type of bullets for the 22-250, 220 swift, even 17 Remington, going over 4000 fps... the only bullet I've ever seen disintegrate in flight was with the 17 Remington, well over 4000 fps...

as far as lighter bullets in your faster twist gun... the only posibility of a barrel expolding, would be from an obstruction, or excessive over charge ( one shot wouldn't do it ), & it's highly unlikely that if a bullet were to come apart, that all the pieces under force of fire wouldn't expell out of the barrel... so B.S. on the barrel problem

I would go as far as to say all "modern factory made bullets" should be perfect enough to withstand the forces of any "normally available" twist rate available in it's caliber, & any "normal" velocity... you will not have problems with factory ammo...

accuracy could be another matter, but I'd go out on a limb & say that the cartridge charge & cartridge length could be tweaked to get the lighter bullet to shoot well ( I'm a handloader, & do it all the time with numerous cartridges )
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Old May 19, 2009, 11:08 AM   #9
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I agree with the bullet speed theory. With that carbine barrel, I'd slow it down a bit, say to 3200. More length gives more time for stabilization as well. With lighter, shorter bullets you don't need that. Reduced speed should change the harmonics enough to match the velocity of a heavier round and make good flight, but I wouldn't expect match grade accuracy.


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Old May 19, 2009, 11:24 AM   #10
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I've seen a .22-250 tear apart bullets in flight. A blue-grey trail. The rifling cut the jacket right off and the lead spun off in the air.

This was back early when I started shooting though, and I don't know the load particulars. It was a varmint rig shooting varmint loads, so it would have been a light bullet being zapped along by a healthy charge in that .22-250 case.
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Old May 19, 2009, 12:35 PM   #11
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It's not just twist that destroys bullets.

I have a 1/9 .223 that won't shoot 55gr varmint bullets like Blitz and SX. The bore it too rough and causes about 1 out of 10 to blue streak and not make it to the target.
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Old May 19, 2009, 02:19 PM   #12
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I have seen 52 grain match hps disintegrate out of the 2nd generation 1/7 twist mini-14.

If the carbine barrel is short it is possible that a cartridge firing a light bullet that would disentegrate in a longer barrel may not do so in a short barrel because the muzzle velocity would be lower.

You would have to be talking 12 or 14 inch though.
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Old May 19, 2009, 02:40 PM   #13
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When bullets disintegrate in flight, they come apart along the rifling marks in their thin jacket, not due to a rough bore. They will disintegrate if spun too fast for the thickness of the jacket. Not all light bullets will disintegrate, some are made to be pushed very fast. But almost all light bullets will "hunt" before "going to sleep" if they are spun too fast, meaning they will spiral slightly for a short distance out of the barrel then stabilize and spin true. That spiralling out of the barrel is what causes many cases of inaccuracy.
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Old May 19, 2009, 07:44 PM   #14
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So... if I was building an Ar...

And planned to use mostly surplus 55gr bullets, what is the
optimal twist rate???
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Old May 19, 2009, 08:15 PM   #15
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If you are talking FMJ then take your pick.
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Old May 19, 2009, 10:07 PM   #16
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1 in 9 is the best all around twist for a civilian shooter who wants to shoot the broadest bullet selection.

1 in 12 is best if you are planning to shoot 55 grain and below at high velocity. i.e. top end handloads.
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Old May 19, 2009, 10:12 PM   #17
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I've loaded 36 grain .223 in my 1:9 16" AR and they work pretty durn good.
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Old May 20, 2009, 01:31 AM   #18
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I have tried 55 grain in my 1:7 M4 upper so far and it shoots sub moa if you can manage it. That was with JHP varmint bullets too. I have some 45 and 50 grain I will try maybe this week or weekend, if I have time.
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Old May 20, 2009, 01:47 AM   #19
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According to Speer manual#14, a bullet fired from a rifle, at 3500fps, with a 1-7" twist, exits the barrel at 330,000 RPM. A rough bore,or a varmint bullet designed for 22 Hornet, with a very thin jacket, or the like, can lose its jacket as it leaves the barrel and be literally torn to pieces. This has never happened to me, but will not harm the rifle, according to Speer.
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Old May 21, 2009, 08:51 PM   #20
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busted

In regard to the exploding barrel image we all have implanted in our mind from Looney Tunes, that is actually a myth. Though not a great fan of the show, Mythbusters proved quite effectively that a barrel won't explode from any accidental obstruction and only from intentionally rigged obstructions.

In regard to the question, is this right? Higher twist rate is better with heavier bullets, and lower with lighter. At risk of stealin the thread, where does barrel length become a negative factor on high twist rates, causing barrel drag? I ask for my own understanding...not rhetorically.
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Old May 21, 2009, 08:54 PM   #21
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http://kwk.us/twist.html
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Old May 22, 2009, 10:08 AM   #22
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I've seen bullets explode at about 50 - 70 yards from my 7.7 twist Krieger barrel many years ago. That was a 223 caliber also. I think they were those really light skinned Sierra varmint bullets.

Wasn't every bullet, but it was several. Not a blue streak either from my vantage point, but rather a grey puff of sorts.
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Old May 22, 2009, 11:20 AM   #23
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I'm of the "go out and shoot a few and see how they perform" school of thought, as opposed to the "take the advice of internet theoretical pontificators" school!
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Old May 22, 2009, 11:26 AM   #24
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I am going by my own first hand experience. It's not a theory at all.

When you shoot at a creek and see a scattering of little fragments hitting the water instead of a single big splash you know something isn't right.
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Old May 22, 2009, 04:49 PM   #25
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AlleyKat has sort of good advice. See how they perform in your rifle. If they don't perform worth a squat, you will know soon enough!



Contrary to what folks would believe, the53 and 52 grain Sierra target bullets shot really well in my 7.7 twist Krieger. So, evidently they were constructed with tougher, maybe heavier gage, jacket material. They really did shoot good.
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