Why a 1 in 14" twist???

[Stover954rr]

Hey Guys,
I was looking a buying a new bolt action .223 target/varmint gun, and I has a beautiful 26" E.R. Shaw barrel. However I noticed that the twist was 1 in 14" !! I know this would be better for shooting hot 40gr rounds. I was always under the impression that for a "target" gun to get any accuracy at distance you would be using a heavier round (62-79gr) and thus a tighter twist.

Now I am confused because I am sure Shaw knows a little more than me when it comes to building a rifle.

~Russ

 

[Gator]

Its for guys who like shooting 55gr bullets?

 

[GunTech]

1:14 is perfect for varminting. As you noted, for anything under about 55 grains, depending on station pressure, temp, etc. Many varmint rifles have 1:14 or 1:12 barrels. 52gn Sierra match kings should do quite well for target shooting at reasonable ranges. You need heavy, high BC bullets to make the 223 perform at long range. Out to 300 yards, the lighter biullets will do fine.

 

[MD_Willington]

I thought it was because you don't want to spin heavy rounds too fast, or you'd end up putting a lot of force on the projectile, that would not be good.

SI wise 5.5mm projectile going 930 m/sec, weighing 0.0040 kg would have 629018 N of Centrifugal Force... would that be close to ripping the projectile apart...?

 

[Vicious-Peanut]

Spin heavy rounds too fast? I think they are OK because of the lower velocity they go at. You don't want to spin light rounds too fast or you can rip the jacket off of them. I think...

 

[Stover954rr]

Yeah, i thought more spin stabilizes the heavier rounds...

 

[blackhawk2000]

1:14 has less spin than 1:7. I believe original spin for M16 was 1:14. They found it did not stabilize enough, so they went with 1:12. 1:12 also is marginal with 55g. With the heavier, and more importantly longer modern bullets, quicker spin was needed. So they went with 1:9, and ultimately 1:7. Again the bullet length is the key here.

 

[GunTech]

The only issue with the 1:14 in the M16 is that it did not adequately stabilize the projectile in arctic condition. In normal conditions it woks just fine. Overspinning a light projectile induces precession and causes the bulllet to yaw away from the normal flight trajectory. In general, overspun bullets won't be 'ripped apart'. They wont be accurate and group sizes will open up.

Ideally, you want to keep the gyrostability factor around 1.25-1.4 for you bullet of choice. 1:14 will barely stabilize 55gn spitzers, but this means the same bullet will become very unstable when transiting media (like flesh) which can make for higher lethality. Even 1:12 is just marginably stable for the 55gn FMJBT M193. for general use, the 1:9 twist is probably a good compormise for any bullet that can realistically fit in an M16 magazine. 1:7 is required to stabilize the new tracer M856 traver. The M855 is fine with 1:9.

 

[Jim Watson]

.22 centerfires typically had 14" twists for many years, meant to shoot 45-55 gr target and varmint bullets. Then the military got into the act. First they found that they needed to go to a 12" twist to shoot cheap 55 gr boattails in cold air, and it has gotten wilder ever since. As per blackhawk and guntech.

 

[MD_Willington]

I figured I probably had it backwards... long day for me... I'm beat, time for a beer and some sleep!

 

[quicktime]

You can easily rip a bullet apart by shooting a light bullet or bullet with a thin jacket at high velocity through a barrel with a fast twist. I have had the problem with a 22-250 and 50 grain bullets. But it is kinda cool to see the bullet dissinegrate about tenty yards down range.

 

[Coronach]

Yeah, the ratio (1:x) is counterintuitive, and will mess you up if you're not paying attention. It translates into 1 revolution in x inches, so the bigger the number the slower the spin rate.

Mike

 

[MD_Willington]

Yep that's the part I couldn't remember Coronach... lol

 

[Harley Quinn]

Well it sure ruins your ability to use the heavier bullets.

244 was a real flaw when it came out and could not compare with the 243 so they then made it the 6mm.

There is a fine line between a reason for wanting. The military came up with a faster twist to over come the situation and 223 just is not in the same velocity grouping as the 22-250 or 220 Swift.

To bad they made the twist so slow in the olden days, for they would have had a better all around, for folks that like to use a rifle (cal) for more than just one item.

But they want you to own 10 rifles and not just a few.

The 30-06 was able to shoot the 100 grain and the 220 Hmmmm

 

[GunTech]

Twists were often slower in the 'olden' days because round nose bullets require a slower twist rate than spitzers. Gyrostbility is effected by length of the bullet, not weight. A round nose bullet requires a slower twist than the same weight spitzer. An all copper bullet requires a faster twist that a copper jacketed lead one because the bullet itself is longer (copper being less dense than lead).

I bears noting that the (in)famous Greenhill formula was developed around roundnose bullets, and is really not appropriate for spitzer and boat tail bullets, which generally require a faster twist.