Range Report Calling 40 Lb. Heads, is there something to this?

[MechanicKid]

So I am re-forming some LC 5.56 brass, reaming primer pockets and etc. when something hits me. There is a lot of talk and suggestions about barrel twist rate vs. bullet weight, not due to weight itself, but bullet shape. Some refer to it as "bearing surface" but generally most are referring to the length of the flats of the profile after the ogive. I hit the books and come up with some numbers. Suggested twist for a .224 55 grain bullet is about 1/9". Some may dispute this, but that is what I gather from most discussions. Now looking at Hodgdon load data, avg. vel for max loads is about 3100 fps. Keep in mind I was just using one row of numbers for comparison, not empirical data. After some number crunching (1 rotation for every 9" of travel at 3100 fps, which slows over time/distance) I came up with 4133 rotations per second. Now, an 80 grain bullet with a suggested barrel twist of 1/7" at an avg of 2700 fps gives us about 4628 rotations per second. Now I know that rotational speed generates cetrifugal force, which in turn imparts stability (see gyroscope). I know that very light bullets in too "fast" of a twist may actually come from together due to jacket thinness but what I am wondering is could you actually stabilize an 80 grain bullet out of a 1/9 twist if you pushed it to just under 3500 fps (matching rotational speed of 1/7" at 2700 fps)? And does this really mean that

1.Bullet stability relies more on rotation speed and sectional density (amount of force produced due to rotation) which can be achieved by increasing/decreasing velocity

2.It could also explain when a bullet "goes to sleep" by the velocity it drops down to at range achieving optimal rotational speed

3. Load calculations for accuracy could be better based off of a combination of velocity and rotational speed?

Any input would be EXTREMELY appreciated!

 

[1SMALLJOHNSON]

Re: Calling 40 Lb. Heads, is there something to this?

Google " Greenhill formula", and this will give you some static numbers to understand. While not a perfect model, it is the industry standard for determining appropriate twist rates. The model will explain that twist rate is determined by density of the bullet and of length of the bullet. However, the formula takes into account the assumption of supersonic velocities (I think).

It's also about the RPM. (Revolutions per minute). The faster you push the bullet, the faster the RPM. Thus, to your point, you COULD stabilize a bigger bullet by pushing it faster, given a constant twist rate. Likewise, if you were trying to push a high sectional density bullet to subsonic velocities, you'd do well to check your stability. You might need more twist than with supersonic velocities, all things else being equal.

As to "going to sleep", I don't know for sure, but a theory is that we're witnessing a balance of forces. One force may be the "buffeting" of the projectile upon muzzle exit, due to muzzle exit pressures upon the base of the bullet. That causes some upset. Second, we've got the centripetal stabilizing force, because the bullet is turning at high RPM. Call it the gyroscopic effect. The initial forces of muzzle exit pressure causing some "buffeting" could eventually be overcome by the gyroscopic effect. Thus, "going to sleep".

Now, you can overstabilize a bullet, stabilize it or understabilize it. When it's understabilized, it's eventually going to turn end-over-end. As it goes thru that process, it presents itself to cross-winds with a much greater "frontal" area to that wind. When that happens, it's a bigger "sail" and is more easily affected by the wind. If you overstabilize a bullet, there's a lot of theories out there. Witness the true affect of a bullet on-target, given a classic 3:00 wind (the wind coming from the right). The bullet does NOT just move left. It moves MOSTLY left, but (right hand twist in your rifling) also moves upward. If you had a left-hand twist in your rifling, the bullet would move left and downward instead. By overstabilizing the bullet, you should likely see an increase in this left-up effect. The other thing that overstabilizing can do is magnify the effect of incorrect balance. If there's a void in the core material, or variation in jacket wall thickness, higher twist rates should magnify that problem. So, does overstabilizing the bullet affect its accuracy? Short answer: It can.

 

[MechanicKid]

Re: Calling 40 Lb. Heads, is there something to this?

interesting formula. Given that C is a constant value (150 or 180 when >2800 fps) it does kind of lend to my theory giving it a formula. THanks for the intel!

 

[Greg Langelius *]

Re: Calling 40 Lb. Heads, is there something to this?

Here's just about everything you'll need to grasp to understand bullet stability and ballistics.

 

[MechanicKid]

Re: Calling 40 Lb. Heads, is there something to this?

Thanks!

 

[gstaylorg]

Re: Calling 40 Lb. Heads, is there something to this?

Also take a look at this, particularly the 2nd equation in the "Corrective Equations" near the bottom:

http://en.wikipedia.org/wiki/Miller_Twist_Rule

Yeah I know, it's Wickipedia, but it addresses the OP's 1st question. Increasing MV will buy you a little bit (but only a little bit) in terms of increased stability. If you look at the equation you will see one reason why. It's an inverse cube function, which means you really have to jack up the MV to much greater than 2800 fps before much of an effect will be noticed in terms of increased gyroscopic stability. If you were using a projectile/MV/twist combination that was close to the ragged edge stability-wise, increasing the MV by a couple hundred fps might make a difference. At one time, I had a .223 with a 21" 10-twist barrel that shot very well with 77 gr SMKs. It probably shouldn't have performed as well as it did with that twist rate, but it did. That rifle generally gave very decent MVs with 77 gr ammo loads, better than would have been predicted from factory (box) MV values. Although I can't scientifically prove it, I've always though that might be why it shot the 77 gr loads so well with a 10-twist barrel.

I've played around with some numbers using this corrective formula; you can easily do the same to convince yourself you'd need a muzzle velocity way beyond what could realistically be achieved in order to significantly increase the stability. There are other factors involved as well. My suggestion to the OP would be to buy and a copy of Bryan Litz' book, "Applied Ballistics for Long-Range Shooting" and read it carefully. You'll only be out the cost of a couple boxes of ammo and the info it contains will be beneficial for any future shooting endeavors. It's well worth every penny.

http://www.appliedballisticsllc.com/Book.htm

 

[MechanicKid]

Re: Calling 40 Lb. Heads, is there something to this?

This kind of information is exactly what I was looking for. Currently being in between contracts I find myself needing to occupy my mind. Thanks guys for the input. I will be actively seeking out these texts and references. Thanks again!