heavier bullet=more twist?

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dieselbeef
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heavier bullet=more twist?

Post by dieselbeef » Wed Oct 28, 2015 9:14 am

maybe a 556-65 vs 223-55.
more twist in a 308 barrel? 1/8 vs 1/9?

say in an ar platform for hoggin maybe..150 yd shots tops
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Re: heavier bullet=more twist?

Post by flcracker » Wed Oct 28, 2015 9:38 am

Generally, yes. But the ballistics experts will say that it's actually more like "longer bullets need a faster twist".

For killing hogs with a 5.56/.223 AR-15, I'd assume you would want to use the heaviest bullets you can find. I did some accuracy testing with Winchester Razorback 64 grn HP lead-free and the Remington Hog Hammer 62 grn TSX HP lead-free in two nearly identical 16" M4-configuration AR-15s - the only difference was the barrel twist.

When I was sighting in the rifles with 55grn mil-surp ammo, their accuracy was pretty much the same. However, both brands of the heavy ammo shot better in the AR with the 1:7 twist as opposed to the one with the 1:9 twist.

At that range, barrel twist won't make much difference in a .308.
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Re: heavier bullet=more twist?

Post by dieselbeef » Wed Oct 28, 2015 9:42 am

so I got it backwards then..heavier bullet is more accurate in a less twist?
were discussing this at work. I said a 556 is a heavier rd..hence more barrel twist.

your saying the 556 should be better with the 1/7 vs 1/9 barrel
im the other gary...

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Re: heavier bullet=more twist?

Post by boomboom » Wed Oct 28, 2015 9:50 am

Gary, it depends on bullet grains not the ammo size. If you are shooting 55gr 5.56/.223 there usually is no problems shooting out of either a 1:7, 1:8 or 1:9. If you jump to a 77gr round, you might find that the 1:7 or 1:8 will stabilize the round and the 1:9 won't. On the other hand if you drop to a 45 gr round, the 1:9 will handle it fine where the 1:7 won't.
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Re: heavier bullet=more twist?

Post by boomboom » Wed Oct 28, 2015 9:52 am

Oops, double post. Fat fingers
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Re: heavier bullet=more twist?

Post by rug357 » Wed Oct 28, 2015 10:45 am

Don't forget that the original M-16 rifles had 1-12" twist rate.
A lot of rifles with 1-12" twist rate will shoot great with 55gr and even 62gr in some cases.
As someone mentioned above, it's not the weight but the length of the bullet (projectile) that requires faster twist rate to stabilize it in flight.

FYI. Don't use "less" and "more" when talking about barrel twist rate as it could be confusing...use "faster" and "slower" twist rates instead.
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Re: heavier bullet=more twist?

Post by dieselbeef » Wed Oct 28, 2015 10:48 am

guys here were talking about it...I had it backwards saying a ''faster twist'' was for a heavier bullet..when in reality its likely the other way around
im the other gary...

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Re: heavier bullet=more twist?

Post by rug357 » Wed Oct 28, 2015 11:02 am

dieselbeef wrote:guys here were talking about it...I had it backwards saying a ''faster twist'' was for a heavier bullet..when in reality its likely the other way around
No, you got it correct.
Heavier/longer bullet requires faster twist rate to stabilize. Like the 65gr hunting bullets works better with 1-9" or 1-7" twist rate barrel.
Lighter/shorter bullet works better with slower twist rate. Like older 55gr FMJ bullets works well with the 1-12" twist rate barrel.
You can have situation where the bullet is rotated too fast to the point where the bullet breaks apart when it leaves the barrel.
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Re: heavier bullet=more twist?

Post by flcracker » Wed Oct 28, 2015 11:43 am

dieselbeef wrote:so I got it backwards then..heavier bullet is more accurate in a less twist?
were discussing this at work. I said a 556 is a heavier rd..hence more barrel twist.

your saying the 556 should be better with the 1/7 vs 1/9 barrel
No, I'm saying that within the same caliber, the heavier/longer bullet typically prefers the faster twist.

It's pointless to compare across different calibers, because the ratio of bullet weight/length to diameter is different. As stated above, 5.56/.223 can use 1:7 all the way up to 1:12, depending on the bullet. 7.62/.308 typically use 1:10 or 1:12.

So, the larger caliber (7.62/.308) typically uses a slower twist than the smaller one (5.56/.223), but not always.

But, within each caliber, the heavier bullet typically does better in the faster twist.

Clear as mud, right? [smilie=cheers1.gif]

Chuck Hawks does a really good job of explaining it here:

http://www.chuckhawks.com/rifling_twist_rate.htm

Here's the math for you smart kids:

Image
and some rin up hill and down dale, knapping the chucky stanes to pieces wi' hammers, like sae mony road-makers run daft - they say it is to see how the warld was made!

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Re: heavier bullet=more twist?

Post by dieselbeef » Wed Oct 28, 2015 1:20 pm

can you get me that formula in common core please so gage can explain it to me [smilie=cheers1.gif]
im the other gary...

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Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe."
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Re: heavier bullet=more twist?

Post by Wulfmann » Wed Oct 28, 2015 3:59 pm

Technically the first ARs were 1/14 but not for long the majority were 1/12 before the modern era Euro M855 ammo changed it to 1/9 (The M855 stabilizes with 1/9 but the M856 tracer required a 1/7 hence the reason the militaries all use 1/7)

It is not the weight it is the bearing surface (which usually means more surface more weight but not always)

Example the Sierra 52gr HPBT-Match has a very long bearing surface for its weight and shoots great out of a 1/7

Plus there is the fact some barrels ignore every rational definition and simply shoot something they should not and what they should shoot well they don't (and vice versa) so testing, testing and more testing

If you look at the bullet (not loaded ammo) some have short bodies and very long points that taper way down from the body while others have long bodies and taper to a shorter point and that tells you the bearing surface difference

BTW in case some may not know the twist rate refers to the distance a bullet travels and completes a 360 degree spin, so a 1/7 twist means it traveled 7" to rotate one time and a 1/12 means it traveled 12" to do a 360 etc etc
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