rifle twist vs bullet weight

[redtab78]

I was reading an article about bullet weight vs rifle twist, and was wondering if there is a general rule of thumb for this, especially when dealing with 5.56/.223 ammo.

I see alot of 1:7 and 1:9 for AR's, but I see a 1:12 for bolt actions (in which the author wrote that a 65g is about the highest he could go reliably with the 1:12 barrel)

 

[Art Eatman]

Heavier bullets are longer bullets. Longer bullets must be spun faster for stability.

Generally, a 1:14 twist works for the 40- to 55-grain bullets.

A 1:10 twist works for bullets around sixty grains or lighter, and also round-nosed 70-grain bullets.

Boat-tail spire-points of 65 and heavier generally are accurate with the faster twists of 1:9 and 1:7.

The lighter-weight bullets can be quite accurate with the faster twists.

 

[oldpapps]

This is a for what it is worth.

Twist formula

Just copy this and paste it into a spread sheet (block C1 is a good option).

=(30*+C5)/((+C7/+C4)^2*+C4^3*+C6/+C4*(1+(+C6/+C4)^2))*(+C8/2800)^(1/3)*((+C9+460)/(59+460)*29.92/+C10)

Then copy and paste this (to make it simple, put the 'Where' in block E2).

Where:
Sg - is the stability coefficient and should not be less than 1.4, but more is ok
C4 - Caliber in inches (.223 for both .222 and .223)
C5 - Bullet Weight in grains
C6 - Bullet Length in inches
C7 - Barrel Twist in inches per turn
C8 - Muzzle Velocity in fps
C9 - Temperature in degrees F (59F normally used)
C10 - Pressure in inches of mercury (29.92 normally used)

This will put the variables in the 'C' column from row 4 to 10. The numbers will have to be typed in to the 'C' column. The location that you put the formula will display the final calculation value.

It is not that hard.

Also, barrels can't read, so most will do what they want, no matter what the spread sheet says. They all are a little different but this does give us something to judge with.

 

[jmr40]

1:7's, I wouldn't suggest anything lighter than 55 gr. 1:12, I wouldn't suggest anything heavier than 55 gr. Everything in between is a compromise. All of my rifles are 1:7. Not a hard fast rule, in individual guns you may find exceptions, but pretty close.

 

[Art Eatman]

One's purpose is always a consideration. I added a lightweight flat-top upper to my "goodies". 20", 1:14 twist as a dedicated varmint rig. So far, one MOA with 55-grain bullets.

I'll stick the original iron-sight upper, 1:10 20", back on if there is a need for serious social work.

 

[ugaarguy]

You can shoot lighter weight copper jacketed lead bullets in a 1:7 twist .223 / 5.56 bbl provided the jacket is thick enough, or you limit the velocity on the thinner jacketed bullets.

Also, I'll issue you my default reminder about copper solids. Lead free ammo bullets are already mandated for hunting anywhere in CA, and are required on public lands / wildlife management areas in more and more states. Copper solids are the most common lead free bullets, and since copper is far less dense than lead those bullets are much longer for their weight. The original Barnes TSX 62 gr 22 caliber bullets requires a 1:9 twist or faster. Their TTSX line are even longer for weight, and the 62 gr 22 caliber TTSX requires a 1:8 or faster twist.

 

[Karamoja]

...was wondering if there is a general rule of thumb for this...

Others already said it: longer bullet lengths (heavier bullets) call for faster twist rates. As for rules of thumb, czech it out... scroll down to the bottom of the page:

http://www.riflebarrels.com/products/caliber_twist_rates.htm

.

 

[MistWolf]

Longer length bullets are not always heavier

 

[Art Eatman]

"Longer length bullets are not always heavier."

For any one given material, they gotta be. But for twist rates, the length is the controlling factor, regardless of materials and comparative weights.

 

[MistWolf]

...For any one given material, they gotta be...

Not always, Art. A VLD (Very Low Drag) bullet will be longer than a round nosed bullet made of the same material, in the same caliber and weight.

If I recall, a RN bullet is easier to stabilize in flight than a SPBT

 

[Bart B.]

If I recall, a RN bullet is easier to stabilize in flight than a SPBT

I don't think so. If it did for any significant amount, then there would surely be a variable for bullet shape in oldpapp's formula posted earlier.

However, low velocity bullets such as .22 rimfire and handgun centerfire match ammo rounds, they've all got round noses and no boattails. Spitzers and boattails don't show up on such subsonic bullets. Like bulbous noses on surface ships (copied form submarine front end shapes), subsonic moving things move faster in low speed bands with such shapes on their front end. I think all ships are still subsonic in speed but they've all got "shiptails."

 

[Jim Watson]

The stability equation above covers both bullet weight and length.
Bullet length is heavily dependent on weight but is also strongly affected by shape.
Which is why the stability equation is more precise than Greenhill, which is based on very blunt projectiles.

Anecdote: My 14" twist .22-250 shoots the short blunt 70 gr Speer pretty well.
It is accurate with a 60 gr flat base soft point spitzer but wild with a slightly longer hollowpoint.

 

[WVRJ]

I have two 223's-an AR with a 1 in 9 barrel and a bolt gun with a 1 in 12 barrel.The AR shoots best with 69 gr BTHP's.The bolt shoots good with either 55 or 60 gr Ballistic tips.But a couple of weeks ago,I tried the 64 gr Nosler Bonded Performance for a deer load in the AR.It wouldn't feed the flat tipped bulets even though they were advertised as being especially for reliable feeding in semi autos.I single loaded them to see how they would shoot anyway.They were at best able to shoot 6 or 8 inch 3 shot groups.I was sure the bolt gun would shoot them even worse.Of course it fed them flawlessly,no problems there.The big surprise came when the first group went into 1.5 at 100 yards.A fluke?No,I've shot numerous groups out of the bolt gun that were all in that range.I would have to guess that the flat tipped,shorter profile of the bullet helped it to stabilize out of the 12" twist.As for the AR-damfino,anybody who could shed some light on this?It usually hovers around 1 to 1.5 MOA with 60 gr ballistic tips.

 

[tactikel]

Remember weight does not always equal length. A boat tail polymer tip bullet is MUCH longer than a flat based soft point. Copper vs lead jacketed changes everything.
My .22-250 14" hates any PTBT bullets but loves 45 gr flat based soft points.