Relationship between bullet speed, twist, and bullet stability

[kmcdonou]

I have a custom, switch barrel rifle (223 and 308) I use for predator control around my chicken coop and yard (skunks, raccoons, fox, coyote), deer hunting, and coyotes in the winter. I want to develop a quieter, less destructive 223 round for predator control around the house and for late fall, early spring racoons/fox. I know I can use Blue Dot powder to develop a load that runs around 2,200 fps, or slightly more than a 22 mag and slightly less than a 22 hornet. I know there can be safety issues if you double charge a Blue Dot load, but I am pretty anal and weight everything. I am not concerned about developing a safe load at my desired velocity.

I want to use a 40 grain V-max for the bullet, as slowed down, and from my reading, I think it will be fur friendly for fox and coon and serve as an effective predator deterrent around he house. My problem seems to be my twist. My 223 barrel is 1:12 and I normally shoot 52-55 grain bullets for coyote.

Using Berger's twist rate calculator with a the 40 grain Vmax, BC at .200, bullet length .686 (from JBM bullet length database), and fps at 2,200, it returns a stability value of 1.08, which is marginally stable. When I move the speed up to 3,600 my value moves up to 1.27, which is still marginal. To get a comfortably stable value of 1.56 at 2,200 fps, I have to move up to a 1:10 twist. I don't have a problem doing that as the cost of another barrel is lot cheaper than a new rifle and scope.

My question is can someone explain the relationship between bullet stability, twist, and bullet speed? I know the formulas for this include bullet speed, but I want a dumbed down version. In general, from what it appears, if you slow down a bullet you need to spin it faster to keep it stable, or vice versa, if you speed up a bullet, you need a slower twist to keep it stable. Is this a reasonable conclusion?

 

[lawman29]

A flat base bullet will be more stable with slower twists and velocity than a boat tail...

 

[Majday]

I don't think I'll answer your question like you want but I did the same thing 15 years ago. I had a ruger mk2 which had at best a 1:12 possible a 1:14. I used Blue Dot as well for means of using on bobcat. My scale doubled on me and blew the stock to pieces and I have brass in face and thumb. Saying that the load I did have shot decent I believe around the same velo as what your looking at getting. I think I was using 50gr spsx at that. They were stable to the target.

I don't think you'll have a problem. I'd just load a few up where you want velo and shoot to see how they fly. Doesn't take much time or components.

I'm no expert in any sense but play it safe like you said and just experiment. I went down the same road- it freaked me out a little, made me more conscious and pay much more attention what I'm doing. I ended up getting a 17 rem and it's dynamite for the tasks your talking about.

 

[lawman29]

was using 50gr spsx

Flat based bullet

 

[dellet]

Over stabilizing is tough to do. Easier just to worry about the bottom end to start.

Bullet shape and weight distribution come into play. That’s why if you read the fine print on the Berger stabilization calculator is says don’t use this for flat base bullets. It will predict a much faster twist than needed.

If you use the JBM calculator, it will be closer because to tries to compensate for the plastic tip. You will probably get a number closer to 1.6. That will probably still be low.

Note if you look at the Berger 40 grain bullet with a length of .609” they recommend a 1/15 twist. Yet the calculator spits out you need a 1/12 to be barely stable at 2500 fps.

I shoot the 40 Vmax at 2800 in a 1/16 22 Hornet, and it’s my go to bullet. That bullet in the Berger calculator would be around .800.

Important Note: The Miller Twist Rate Formula isn’t accurate with flat based bullets, so we recommend you view our flat based bullet twist rate chart to identify what the minimum twist rate is for all of our flat based bullets.

 

[gpoldblue]

I have a custom, switch barrel rifle (223 and 308) I use for predator control around my chicken coop and yard (skunks, raccoons, fox, coyote), deer hunting, and coyotes in the winter. I want to develop a quieter, less destructive 223 round for predator control around the house and for late fall, early spring racoons/fox. I know I can use Blue Dot powder to develop a load that runs around 2,200 fps, or slightly more than a 22 mag and slightly less than a 22 hornet. I know there can be safety issues if you double charge a Blue Dot load, but I am pretty anal and weight everything. I am not concerned about developing a safe load at my desired velocity.

I want to use a 40 grain V-max for the bullet, as slowed down, and from my reading, I think it will be fur friendly for fox and coon and serve as an effective predator deterrent around he house. My problem seems to be my twist. My 223 barrel is 1:12 and I normally shoot 52-55 grain bullets for coyote.

Using Berger's twist rate calculator with a the 40 grain Vmax, BC at .200, bullet length .686 (from JBM bullet length database), and fps at 2,200, it returns a stability value of 1.08, which is marginally stable. When I move the speed up to 3,600 my value moves up to 1.27, which is still marginal. To get a comfortably stable value of 1.56 at 2,200 fps, I have to move up to a 1:10 twist. I don't have a problem doing that as the cost of another barrel is lot cheaper than a new rifle and scope.

My question is can someone explain the relationship between bullet stability, twist, and bullet speed? I know the formulas for this include bullet speed, but I want a dumbed down version. In general, from what it appears, if you slow down a bullet you need to spin it faster to keep it stable, or vice versa, if you speed up a bullet, you need a slower twist to keep it stable. Is this a reasonable conclusion?

Years ago I found a load in an old American Rifleman magazine using 6gn of Unique with a 45gn bullet for my Rem 788,.223, 12 twist worked beautifully. Crono'd about 1950fps shooting with 5 shot groups @ 100yds of about .6" ....nice.....about like a .22 mag except better accuracy.....only a suggestion.
Good luck and stay safe.

 

[sw282]

Cast Boolits

This is the dedicated cast bullet forum!

castboolits.gunloads.com

cast boolits forum is a source for reduced loads. l shoot IHMSA pistol. Quite a few shooters use cast
loads out to 200meters in 22Hornet, 221fb, 30 Herrett, and other 224, 308, 6mm, 7mm rifle cartridges

 

[kmcdonou]

Bullet shape and weight distribution come into play. That’s why if you read the fine print on the Berger stabilization calculator is says don’t use this for flat base bullets. It will predict a much faster twist than needed.

I forgot about flat based bullets requiring a different calculator, but according to the appearance on Hornady's site, the 40 grain Vmax is a boattail bullet, so the calculator should be accurate.

I shoot the 40 Vmax at 2800 in a 1/16 22 Hornet, and it’s my go to bullet. That bullet in the Berger calculator would be around .800.

I am confused how your 1/16 twist barrel, with a 40grain Vmax at 2800 fps is stable. You seem to imply the JBM bullet database does not compensate for the plastic tip, which will make the overall length of the Vmax closer to .800. Putting that info into the calculator, with the 1/16 twist, .200 BC, and 2800 fps gets me a value of 0.46, highly unstable. Yet, you say it is fine for you. What am I missing?

 

[3Ackleys]

I shoot the 40 Gr. Vmax @ 2500 Fps, in my Sako 222 w/14 twist. I use a starting load of H4895. Shoots excellent.

 

[Forum Boss]

Quote: "In general, from what it appears, if you slow down a bullet you need to spin it faster to keep it stable, or vice versa, if you speed up a bullet, you need a slower twist to keep it stable. Is this a reasonable conclusion?"

If you speed up a bullet, it will generally remain stable even in a faster twist barrel (within reason).

I've shot 70gr 6mm bullets in a 1:7.5" twist barrel chambered for 6BRA and they shot perfectly well. The reason to do this is:

1. One barrel for all weight bullets of a particular caliber.
2. High RPM offers more explosive effect when shooting varmints -- quite noticeable.

The down-side (in some situations), is that extremely high rpm can cause bullets to break up in flight. That has been observed. But it's not something most folks need to worry about.

 

[jelenko]

Each bullet [based on it's shape, weight and length] requires a minimum number of RPM's to be stable.

At a given muzzle velocity, increasing twist rate will increase the RPM of a bullet.

At a given twist rate, the RPM is proportional to the muzzle velocity [faster mv => higher RPM].

All I know about the 'marginal stability' range comes from Berger:

Bullets flying with SG's between 1.0 and 1.5 are marginally stabilized and will fly with some amount of pitching and yawing. This induces extra drag, and reduces the bullets effective BC. Bullets in this marginal stability condition can fly with good accuracy and precision, even though the BC is reduced​

 

[K22]

The 13th Edition of the Speer Reloading Manual has reduced loads for a 12", 223 Rem. with velocities in the 1,900 to 2,100 f/s range for bullets ranging from 40 to 55 grains. The powder listed in XMP5744 which is mfg. by Accurate Power Co. Not sure they make it anymore.

 

[kmcdonou]

Bullets flying with SG's between 1.0 and 1.5 are marginally stabilized and will fly with some amount of pitching and yawing. This induces extra drag, and reduces the bullets effective BC. Bullets in this marginal stability condition can fly with good accuracy and precision, even though the BC is reduced

If this is true, I shouldn't even bother worrying about stability, particularly if I am shooting under 100 yards. The Berger calculator with my 1:12 twist barrel, 40 grain Vmax, and 2,200 fps gives a value of 1.08, which is marginally stable. So, as long as my load is accurate and I am shooting at shorter distances, is there any reason to be concerned? The calculator says I should use a 1:10 twist for this bullet and speed.

 

[jelenko]

value of 1.08,

Personally, I'd want more margin than that. But, you can always try it out.

 

[dellet]

I forgot about flat based bullets requiring a different calculator, but according to the appearance on Hornady's site, the 40 grain Vmax is a boattail bullet, so the calculator should be accurate.



I am confused how your 1/16 twist barrel, with a 40grain Vmax at 2800 fps is stable. You seem to imply the JBM bullet database does not compensate for the plastic tip, which will make the overall length of the Vmax closer to .800. Putting that info into the calculator, with the 1/16 twist, .200 BC, and 2800 fps gets me a value of 0.46, highly unstable. Yet, you say it is fine for you. What am I missing?

You’re right I confused the 40 VMax with the 45 Hornet bullet which is the flat base.

Regardless, Berger formula does not seem to compensate for plastic tips either.

The 40 VMax is .688” including the plastic tip, JBM compensates for the .140” tip. And spits out a different SG number than Berger.

According to JBM the 40 VMax is 1.5+ at 1800 fps in a 1/2.

Hornady actually has data for the 40 in a 1/16, and I have used it with no problems.

 

[6ShotsOr5?]

Stability is not a one and done issue. It changes as the bullet conditions change. Velocity slows and spin rate decays as the bullet flies down range. If your stability is 1.08 at muzzle conditions, it may be unstable after the bullet has traveled 100 yds. You can easily get a velocity at 100 yds for your application. Spin rate decay is more difficult - I’m not aware of any good tools for that. Just redo the calculation at 100 yd velocity as a next step, and remember the answer is a little optimistic because you aren’t including spin rate decay.

 

[Herzo]

Just a suggestion, you might try Trail Boss (if you can find it) in either caliber. I would imagine that the 30 caliber100 grain plinkers or 110 grain carbine bullets might perform well in the velocity range you are looking for. Just checked the sg for the 110, it is in double digits for a 1:10 or 1:12 twist.

The nice thing about Trail Boss is that it is virtually impossible to get into trouble with it.
IIRC, I use 17 grains in my 44-77 to drive a 385 grain cast lead bulletin 1100 fps.