Understanding bullet weight and twist


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BNAllen
January 23, 2010, 03:18 PM
I have two AR15's. One is a target rifle, the other I use to hunt coyotes. Both have a 1 in 7 twist barrel. The faster twist barrels do not group lighter bullets as well as they do heavier bullets (55 gr versus anything north of 70 gr). Can someone guide me to a good resource to understand the science behind this.

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rcmodel
January 23, 2010, 03:37 PM
Google Greenhill formula.

rc

jlg
January 23, 2010, 03:59 PM
The longer the bullet the faster it needs to spin to stabilize. Since the diameter is always the same, the heavier bullets are generally longer.

Think about a football. It's much easier to throw a spiral with a small, light football. With a regulation size ball you really have to put some spin on it to keep it from flying like a wounded duck.

243winxb
January 23, 2010, 04:23 PM
http://www.stevespages.com/page8e.htm Greenhill formula

BNAllen
January 23, 2010, 05:33 PM
I understand that heavier (longer) bullets require more twist to stabilize, however, why do my faster twist barrels (1/7) struggle to group a lighter (shorter) bullet? It sems to me that a faster twist barrel should stabilze and group a shorter bullet equally well, provded that muzzle velocity is not too great casuing the bullet to spin apart once it leaves the muzzle.

I'll check the greenhill formula.

243winxb
January 23, 2010, 07:52 PM
If your 55gr bullets are FMJ with lead exposed base, this would be the cause if compared to a soft point bullet or match bullet. Or you gun does not like the brand of 55gr bullet, try a different brand, sierra or Berger. Both makes do list the twist rates for some or all of there bullets. http://www.6mmbr.com/223Rem.html

Kernel
January 24, 2010, 12:08 PM
Greenhill is so 1878. :p You can do better.

http://www.thehighroad.org/showthread.php?t=494440

gearheadpyro
January 24, 2010, 12:56 PM
Interesting links Kernel. I bookmarked both pdf's and will do my best to wrap my head around the math.

Bart B.
January 24, 2010, 01:23 PM
Lighter bullets spun too fast tend to wobble more from centrifugal forces caused by even the smallest unbalance. Every bullet fired is unbalanced to some degree so they need to be spun only fast enough to keep their long axis parallel to the trajectory they're on.

BNAllen
January 24, 2010, 06:04 PM
On the road today ... just checked the thread. Tonight, after dinner and football, I look forward to reviewing the math. Thanks!

benzy2
January 24, 2010, 07:51 PM
"Lighter bullets spun too fast tend to wobble more from centrifugal forces caused by even the smallest unbalance. Every bullet fired is unbalanced to some degree so they need to be spun only fast enough to keep their long axis parallel to the trajectory they're on."

Any proof to this or is it just a claim that makes sense? I have heard it before, but I have also heard some of the better shooters out there say the only risk is spinning really light bullets to the point the lose their jackets. I understand a few other benefits of using just enough twist, but I'm not sure we are really going to see that big of a change in RPM to make much difference, and certainly not something I would think would "ruin" groups from a quality rifle.

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