Barrel Twist, RPM, and Miller Stability Calculator

[Wyo7200]

I was bored today and decided to discover the commonality of each "most accurate" load from Nosler's Load data. Made up this excel spread sheet to calculate the RPM, Miller Stability and suggested Barrel Twist in one place. All you have to do is enter the bullets measurements and chrono data. Maybe it can help someone estimate a stable combo without burning components, and maybe answer some of the why's we all have as reloaders.

I'm no geologist, but what surprised me was the somewhat consistent Miller Stability range within each caliber. Changing the twist to suggested rate- gets the Miller Stability very close to 1.5 (as recommended by Brian Litz). Looks like the next case loadings, I'll be testing some other components to possibly stretch the legs out and get better long range accuracy on some calibers.

Here's a screen shot: Left column describes what the load is.

real world results (approx off the top of my head): 300 blkout 1"-2.5" at 100. 7mm Rem .33" at 100, .75 at 200, 1.75" at 400. 6.5 Creed .5 at 100, 1" @ 200.

I'll update this with a link to be able to download the excel file once I put the equations into the worksheet VBA module so it doesn't get corrupt. Maybe add an OBT column as well...

 

[Guy Miner]

Cool.

 

[SJB358]

Has anyone ever found a downside to extra twist (within reason) beside thin jacketed varmint bullets?

Seems to me that most of the hunting calibers I use benefit from extra twist with tough hunting bullets. Maybe not so much twist, but RPMs. Seems like the bullets recovered that retain heavy front ends looks like props on a boat.

Cool chart. I'm glad I'm not the only one who thinks about this sorta stuff! :lol:

 

[jimbires]

Scotty , when using a bullet capable of the extra rpm's , I'm going to say the down side to extra twist would be spin drift . but this should only be a problem when you are shooting longer distances , I'll guess 800 + yards .

 

[SJB358]

Got ya. That makes sense. I've never calculated spin drift. Might be interesting to see where and how much it differs from a "normal" twist.

 

[Wyo7200]

my 7.62 168's have a spin drift of 5" at 700 yards. 300 blkout 125's is 4" at 450 yards. 25-06 110's is 4.5" at 700. I think this can demonstrate better than I can speak to what "can" happen if there is too much twist:

http://www.outdoorlife.com/articles/guns/rifles/2007/09/testing-twist-myth

I started this little venture because I was thinking of getting a different caliber and replacing the .300 blkout barrel with a wildcat called 30 American. I’m not thrilled with the accuracy out of the .300 with the lighter bullets, and lobbing heavier bullets doesn’t appeal to me. Sure, the lighter 308’s can group well at 100 and maybe 200, but I want to go further. I want something better than an average 2” at 100 yards and better than 4” on a good day at 200-300 yards. Something that the blkout was not made for. So, before I drop $400 on a barrel, I want to see if I could get better accuracy at longer range. They offer three twist rates- 7, 8.5, and 10 in the 30 A. I got to thinking, my blkout is 8.5, what difference would the added velocity make? Comparing what info I could and seeing that the MS and RPM’s are nearly identical, I would just be burning $ and more powder. No surprise, a slower twist (the blue lines) is needed to get the most stability out of the 110’s and 125’s.

Maybe I’ll ask if they can do custom twist.

I’ll have to update this with another Miller stability calculation for plastic tipped bullets. I don't think it will help with the blkout though.

http://arxiv.org/ftp/arxiv/papers/1410/1410.5340.pdf

http://arxiv.org/ftp/arxiv/papers/1410/1410.5737.pdf

 

[Wyo7200]

Another source for reading. Had to sort through my history to find it.

http://www.shootingtimes.com/ammo/ammunition_st_twistrates_200809/

"Over-stabilization as a result of a too-fast twist rate can occur, resulting in a bullet that travels along its downward arc with its tip pointing skyward, exacerbating wind drift and hastening velocity loss. Ideally, the tip of a properly stabilized bullet should tip downward as the bullet begins its downward arc. Clearly, an over-stabilized bullet is not conducive to accuracy, but it is only noticeable in extreme cases of over-stabilization and at very long range. You’d never notice it at hunting distances."

 

[jimbires]

Scotty , at the distances I shoot my rifles I've never had to be concerned with spin drift . I think the target shooting guys only use enough twist , no extra .


Wyo , I need to read the links you put up . thanks for the info.

 

[SJB358]

So I've read that the long heavy bullets actually lose BC if they aren't stabilized quickly in flight due to them yaw'ing as they get stabilized so it seems like there is a fine line between enough and too little and then you've gotta factor in the elevation your shooting at to I guess.

 

[Dwh7271]

Good info and articles, thanks!
How does or how do you use this calculation with a gain twist barrel, or can it be done?

 

[Wyo7200]

Good info and articles, thanks!
How does or how do you use this calculation with a gain twist barrel, or can it be done?

I honestly don't know. I suppose there would be an exponential factor in there somewhere. I've only heard of gain twist being tried, but not catching on so lilja(?) stopped making them.

I know this is nothing new. But I got tired of switching between 3 websites trying to figure this all out.

 

[Wyo7200]

I slept at an Air Force Inn last night so I should be on top of this today. Hope the couple of Margarita's don't get in the way. I worked in the calculation for plastic tips in a new column. Pretty neat to see it like this.

I've decided to scrap the 16" .300 blkout barrel and go with a 1:12 twist 7.62x40 WT (its .300 blkout supersonic done right) for the 110's-150's. Didn't feel like going Russian x39 or just increasing the RPM by trying the 30 American.

Link to download the excel sheet here: https://www.dropbox.com/s/zoe4fl6m7hoxb6b/Reloading Calcs.xlsm?dl=0

 

[Dwh7271]

Thanks!
You did a great job on that.
Stuck it into my Dropbox. Thanks again!

You know, I hadn't thought about it, but we could stick QL factors, burn, weighting etc in Dropbox also and use it that way.

You doing an 11 twist on the new barrel?

 

[RiverRider]

Another source for reading. Had to sort through my history to find it.

http://www.shootingtimes.com/ammo/ammunition_st_twistrates_200809/

"Over-stabilization as a result of a too-fast twist rate can occur, resulting in a bullet that travels along its downward arc with its tip pointing skyward, exacerbating wind drift and hastening velocity loss. Ideally, the tip of a properly stabilized bullet should tip downward as the bullet begins its downward arc. Clearly, an over-stabilized bullet is not conducive to accuracy, but it is only noticeable in extreme cases of over-stabilization and at very long range. You’d never notice it at hunting distances."

On the money.

My own rudimentary examination of the over-stabilization question seems to indicate that it would never become a problem until the range approaches about 2000 yards with a typical modern high velocity rifle cartridge. Some who are recognized as experts say that over-stabilization is a myth, but I believe they say that because they don't want to explain things. Certainly the phenomenon will never affect the typical hunter who won't shoot much further than 400 yards or so, but over-stabilzation is NOT a myth.

 

[Wyo7200]

Thanks DWH. Im still working on embedding the equations and adding the OBT node calculator into the workbook.

I went with a 12 twist. Wilson Combat is the only one that is making the caliber. I don't expect it to go mainstream with all of the other 5.56 conversions on the market, but it fits for what I want to do. I was surprised and happy to see it in QuickLoad. Now the wait begins for parts!

 

[Wyo7200]

On the money.

My own rudimentary examination of the over-stabilization question seems to indicate that it would never become a problem until the range approaches about 2000 yards with a typical modern high velocity rifle cartridge. Some who are recognized as experts say that over-stabilization is a myth, but I believe they say that because they don't want to explain things. Certainly the phenomenon will never affect the typical hunter who won't shoot much further than 400 yards or so, but over-stabilzation is NOT a myth.

That's some good info. Do you think a low velocity cartridge such as the blackout will show signs earlier? My experience with 110's and 125's flight path peak near 150-175 yards, I wonder if they start the tip skyward destabilizing act- especially the butt heavy 110's- near that yardage and groups at 200-300 (stretching it far for the .300 blk) will be a spread. Groups really well at 100, but has a life of its own after that.

 

[RiverRider]

On the money.

My own rudimentary examination of the over-stabilization question seems to indicate that it would never become a problem until the range approaches about 2000 yards with a typical modern high velocity rifle cartridge. Some who are recognized as experts say that over-stabilization is a myth, but I believe they say that because they don't want to explain things. Certainly the phenomenon will never affect the typical hunter who won't shoot much further than 400 yards or so, but over-stabilzation is NOT a myth.

That's some good info. Do you think a low velocity cartridge such as the blackout will show signs earlier? My experience with 110's and 125's flight path peak near 150-175 yards, I wonder if they start the tip skyward destabilizing act- especially the butt heavy 110's- near that yardage and groups at 200-300 (stretching it far for the .300 blk) will be a spread. Groups really well at 100, but has a life of its own after that.

No. A bullet does not "begin to tip skyward," in the middle of its trajectory.

The spread you're describing is most likely due to wind. Those light .30-cal bullets have very low BCs and are very succeptible to the effects of wind---or even just breezes.

Over-stabilization is not something any of us are ever likely to encounter. I rarely shoot at anything over about 400 yards myself, but even at 1000 yards the effects of it should never be detectable. At 2500 yards, maybe, but I don't think I can even see anything that far away and sure as heck won't be shooting at it if I did.

 

[hellrazor]

A bullet will not tip up. As long as it is still spinning gravity is going to be working to pull it down in a typical arc.

 

[Ridgerunner665]

Over stabilization only matters with high angle fire....as in artillery rounds.

No typical rifle round will ever have a problem with it... Not even at extreme long range.

Bryan Litz has tested this... I trust his results.

 

[RiverRider]

Over stabilization only matters with high angle fire....as in artillery rounds.

No typical rifle round will ever have a problem with it... Not even at extreme long range.

Bryan Litz has tested this... I trust his results.

I believe that's true, but a lot of people interpret his pronouncement to mean that over-stabilization does not exist. It certainly COULD happen, but things are not done in such a way as to make this a common problem. The USMC and US Army do not barrel weapons with excessive rates of twist, and that's typical, and in our civilian weapons we run into insufficient rates of twist if there's any problem at all with it.

I did some rough analysis of the world-record longest confirmed sniper kill. It was at something like 2800 meters. At that range, IF the bullet was over-stabilized, then there would be some problems. I don't recall my exact figures but I estimated that the angle of departure was something like 5-1/2 degrees. At that angle, a bullet that did not yield to the center of pressure would be descending with an angle of attack near 11 degrees toward the end of its flight, and in that scenario all bets are off in terms of predicting trajectory and point of impact.

I think all the attention is given to under-stabilization because that's what we run into in the real world. I've never found any information on just how much twist it takes to make a projectile remain rigid in space regardless of aerodynamic forces. I guess its just one of those questions that never gets asked.