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.223 Rifle Twist Rate

This is a discussion on .223 Rifle Twist Rate within the Range Reports forums, part of the Firearm Forum category; From Iowegan’s Library post : .224 / 5.56mm 1:16 up to 55 grains, 4300 fps or more 1:15 up to 55 grains, 4100 – 4300 ...


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Old February 24th, 2009, 01:56 PM   #1
 
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.223 Rifle Twist Rate

From Iowegan’s Library post:

Quote:
.224 / 5.56mm 1:16 up to 55 grains, 4300 fps or more
1:15 up to 55 grains, 4100 – 4300 fps
1:14 up to 55 grains, less than 4100 fps
1:12 55 – 63 grains
1:9 63 – 70 grains
I’m using a 55 gr round for my Mini-14 Target (1:9).

Q: How will using a smaller grain effect ballistic performance at 25 yrds, 50 yrds, 75 yrds, 100 yrds?

Or

Q: At what point and to what degree does using a smaller grain make a difference?



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Old February 25th, 2009, 04:11 AM   #2
 
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Well, I was thinking the same thing when I got my .223, also with the 1:9 twist barrel. After rigorous (un)scientific testing, I've found that at least my gun likes anything from 40gr V-Max to 75gr BTHPM. Both seem to group just a bit under an inch @ 100 yds. But as they say, YMMV
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Old February 25th, 2009, 09:52 AM   #3
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Rifle twist rates are "optimized" for best down range bullet stability based on bullet length. Because length is related to weight, you can pretty much use the standard chart for determining the best match. The chart assumes a full length barrel (24") to get advertised velocity.

The spin rate of a bullet is derived from two factors ... the velocity of the bullet and the twist rate of the barrel. If you have a shorter barrel the twist rate has to be a bit faster to achieve the optimum bullet spin rate because the velocity is slower. When bullets spin too slow, they will lose stability and start to tumble somewhere downrange. When shooting at distances short of where the bullet loses stability, accuracy will still be just fine however at longer distances, the bullet will begin to wobble and eventually start to tumble, which causes accuracy to go down the tubes. If bullets spin too fast, they are said to be "over stabilized", which really means they too lose stability at some distance and start to wobble and keyhole, just like bullets that are too heavy. Friction from the air slows the bullet spin rate and velocity, but not always at the same ratio. As long as the proper spin-to-velocity ratio is maintained, the bullet will stay stabilized. If either the velocity or spin rate slows down where the ratio is not proper, the bullet will become unstable irregardless of weight. Lighter weight bullets are less aerodynamic (lower ballistic coefficient) so they tend to lose their "ratio" at shorter distances.

As you probably know, rifle bullets have a heavier base than nose so they are naturally unstable. The same issue would be present if the nose were heavier than the base. If the bullet was perfectly symmetrical where the center of gravity was exactly in the middle of the bullet, stability would not be an issue at any distance or velocity.

Most any weight bullet will maintain stability (and accuracy) at closer distances but it's at longer distances where instability starts being a problem. With a 223 Rem shooting a 55 gr bullet, a twist rate of 1:14 will maintain stability for about 150 yards. A 1:12 twist rate will extend the stability to about 400 yards, maybe more with a match grade bullet. Any increase in twist rate will not improve the stability of a 55 gr bullet and in fact a twist rate faster than 1:10 will destabilize the bullet, making is max accuracy range shorter. A lighter 45 gr bullet will also optimize at about 400 yards but with an optimum twist rate of 1:14 and a max twist rate of 1:12. Heavier bullets, such as a 62 gr require a faster twist of about 1:10 and will maintain stability to at least 500 yards. When you get up to 70 grain bullets, you will need a very fast twist rate of 1:9 but because the heavier bullets have a much better ballistic coefficient, you can expect stabilization out to 750 yards.

As mentioned before, velocity is a key factor for spin rate. Chamber pressure limits how fast you can drive a bullet so lighter bullets can be driven much faster than heavier bullets without exceeding chamber pressure limits. If you shoot heavy bullets, the velocity is lower so you need a faster twist rate to end up with the proper downrange spin rate.

Bullets are not created equally. The less expensive "bulk" bullets or the cheaper loaded cartridges typically have weight variances. If you weigh a batch of bulk bullets, it's pretty normal to see a couple grains variation from heaviest to lightest. This means the lighter weights have tiny voids (bubbles) internally and are not perfectly balanced. The match grade bullets will typically all weigh within .1 grains so they tend to be nearly perfectly balanced. When you shoot bullets with voids, they tend to wobble like a whiffle ball so your groups are going to spread ... the farther the distance, the more they will spread.

Your Mini-14 Target has a 1:9 twist rate and a 22" barrel. That means your barrel won't quite get to advertised velocity and will be about 100 fps slow. So .... your shorter barrel will spin a bullet about the same as a 1:10 twist rate in a 24" barrel. That means your optimum bullet weight should be about 60 to 65 grains and should stabilize out to at least 600 yards. Lighter bullets will still be quite accurate at shorter distances ... 300 to 400 yards so it really depends on how far you plan to shoot. There's no sense in wasting money on heavier match grade bullets if you are only shooting at 300 yards or less.

Edited to add: Lighter bullets with higher velocities tend to shorten barrel life considerably. I wouldn't go below 55 gr with the Mini.

Last edited by Iowegan; February 25th, 2009 at 10:42 AM.
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Old February 25th, 2009, 11:56 AM   #4
 
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Wow, Iowegan is a walking, talking encyclopedia on anything for guns. I'm afraid if I were to talk to him too much my head would explode
Iowegan, thanks for all your contributions to this forum, you sir are a rock star!

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