How fast a twist rate is to fast

[lal357]

Like it says how fast is to fast im looking at a new barrel and ran the twist rate calculation from bergers site for 4 possible bullets i would shoot . These will be shot in a 300 win mag 30" barrel
230gr A-TIP bc .414 at 2750 only used when going out past 1 mile
10t 1.18 sf
9t. 1.46 sf
8.5t. 1.76 sf

225eldm bc .391 at 2850 main bullet choice
10t. 1.87 sf
9t. 2.31 sf
8.5t. 2.59 sf

212eldm bc .336 at 2950
10t 1.48 sf
9t. 1.48 sf
8.5t 2.04 sf

215 berger hyb bc .361 at 2950
10t 1.50 sf
9t. 1.85 sf
8.5t. 2.07 sf

 

[Evan]

Rough rule is to never spin the bullet faster than 300,000 rpm. 275,000 is safer though.

 

[Ned Ludd]

As a general rule, you want to run the minimum twist rate that nets you the full intrinsic BC of a given bullet at the velocity you can realistically drive it, under a generic set atmospheric conditions that represent those under which you shoot most often.

Berger's twist rate calculator is set up such that a gyroscopic stability factor (Sg) of 1.50 is necessary to achieve the full intrinsic BC of a given bullet. In my opinion, that is a pretty conservative value. I have used a number of barrel/bullet combos that for which the twist rates were "slow" according to that metric, and would yield Sg values when using the Berger Twist Rate calculator from as low as 1.35 to 1.37, up to the mid/upper 1.4's. They all worked just fine.

Until the Sg of a given twist rate/bullet combination drops down into the ~1.1 range, it will usually be sufficiently stabilized to prevent key-holing (flying sideways) at or before reaching the target. However, giving up a very small percentage of the total intrinsic bullet BC is not something that you can readily measure, let alone reliably shoot the difference. If you try to stay in the Sg = 1.4 to 1.6 range, you will be reaping most of the BC benefit of a given bullet, without being in danger of grossly over-spinning it. As Evan suggested above, grossly over-spinning bullets may have some marginal effect on accuracy, but can also most definitely have an effect on jacket/bullet integrity. I've run a few fairly fast twist .224 and .30 cal barrels. The precision seemed just fine with both, but I've lost a few .224 bullet jackets on the way to the target, not something you want to have happen in the middle of a match.

 

[lal357]

The one that bothers me is the 225 eldm in a 8.5 t wist at 2.59 and the 230 atip at 1.46 in a 9twist i will mostly be shooting the 225 eldm and only use the a tips when i go to blakley to shot past a mile

 

[r bose]

Like it says how fast is to fast im looking at a new barrel and ran the twist rate calculation from bergers site for 4 possible bullets i would shoot . These will be shot in a 300 win mag 30" barrel
230gr A-TIP bc .414 at 2750 only used when going out past 1 mile
10t 1.18 sf
9t. 1.46 sf
8.5t. 1.76 sf

225eldm bc .391 at 2850 main bullet choice
10t. 1.87 sf
9t. 2.31 sf
8.5t. 2.59 sf

212eldm bc .336 at 2950
10t 1.48 sf
9t. 1.48 sf
8.5t 2.04 sf

215 berger hyb bc .361 at 2950
10t 1.50 sf
9t. 1.85 sf
8.5t. 2.07 sf

Is it possible that your numbers are incorrect? 225 ELD-M 2.31sg AND 212 ELD-M 1.48sg both in a 9 twist? 225's should have a lower SG, Right?

 

[lal357]

That's what I got from Bergers site

 

[Ned Ludd]

The Sg numbers you get from Berger's site may not be exactly correct because the Hornady bullets have plastic tips that the Twist Rate Calculator doesn't take into account. The sectional density of the plastic tip is different than a copper-jacketed OTM bullet. There have been a couple threads here about the effect of the plastic tip on twist rate calculations and Sg, but I don't recall the exact conclusions. However, I'd guess the values you generate are probably close enough to for a rough approximation/estimate.

 

[FrankG]

I’m running a 9 twist in 300PRC. I’ve shot the 225 ELDM’s, 230SMK and the 230 ATIP’s. I haven’t shot the ATIPS past 430 yards but the 225 and 230’s out to a 1K all with no issues. I’m running them at 2900fps.

The 225’s will fly in a 10 with no issues. The 230smk and the 230 ATIPS according to my calc need a little faster than a 10 twist. I’d run at least a 9.5. The 9 twist isn’t going to hurt you.

Your not spinning these bullets to fast in a 9 twist.

Later, Frank
Bartlein Barrels

 

[FrankG]

Rough rule is to never spin the bullet faster than 300,000 rpm. 275,000 is safer though.

300K rpm is my line in the sand where bullet failures start or should I say go up. The failure rate is higher in smaller calibers like .224, 6mm and say up to 6.5.

That being said I know of .30cal bullets shot in 5 twist and faster barrels and failure rate is a problem. That was special testing with special bullets. The average guy will never be running a fast twist like that.

Velocity x 720 and then divide by twist will give you bullet rpm.

Later, Frank
Bartlein Barrels

 

[lal357]

Thanks frank for the math doing the math i wont be over 260,000 rpms

 

[Geno C]

300K rpm is my line in the sand where bullet failures start or should I say go up. The failure rate is higher in smaller calibers like .224, 6mm and say up to 6.5.

That being said I know of .30cal bullets shot in 5 twist and faster barrels and failure rate is a problem. That was special testing with special bullets. The average guy will never be running a fast twist like that.

Velocity x 720 and then divide by twist will give you bullet rpm.

Later, Frank
Bartlein Barrels

How about 300gr Berger’s going 33-3400fps out of an 8 twist. I’ll hopefully see this winter if the big bullets can handle it

 

[Brady W]

How about 300gr Berger’s going 33-3400fps out of an 8 twist. I’ll hopefully see this winter if the big bullets can handle it

The 300 grain .338 OTMs at 3400 FPS really need a 1-12 twist, but you are spanking that bullet so you can get away with it being slower. The 1-10 twist or faster is better at 3000 fps and slower.

That being said, at 3400 FPS the OTMs are spinning at 306,000 rpms, which is over the recommended maximum. At 3000 FPS that 1-8 is running those bullets at 270,000 FPS.

 

[FrankG]

How about 300gr Berger’s going 33-3400fps out of an 8 twist. I’ll hopefully see this winter if the big bullets can handle it

What Brady said!

I think your o.k. at the 306k rpm with that bigger bullet I don't feel failure will be a huge problem but you should be aware of it.

It's also going to come down to bullet design as well and how it's made.

Later, Frank

 

[dellet]

Speaking as someone who has experience with over spinning bullet failure,

Don't forget about engraving depth on the bullet jacket, barrel heat and friction from heat in flight.

I had loads that were apparently on the edge during load work up and sighting in, that came apart at longer distance and rapid fire.

A couple flyers, then complete misses, followed by a strange sound firing.

Having to pay for a recore to a suppressor made me a firm believer in staying well below 300K rpm.

 

[anschuter2013]

All those calculations and figuring is fine, but One can easily tell if the twist rate is too fast if you squeeze the trigger and the barrel unscrews itself and takes off sideways into the neighboring bench. That's all I know.

 

[fyrewall]

The Sg stuff.

Here is the formula for use in Excel - the Miller Twist Estimator

For multiple Sg calculations, Column H, rows 36,37,38,39,40,41,42, 43. The Sg may be placed any where but H44 is easiest.

=(30*H37)/((H40/H36)^2*H36^3*H38/H36*(1+((H38-H39)/H36)^2))*(H41/2800)^(1/3)*((H42+460)/(59+460)*29.92/H43)

= Sg (H44)

H36 = bore diameter
H37 = bullet weight
H38 = bullet length
H39 = length plastic tippy thing
H40 = twist, inches per complete turn
H41 = muzzle velocity in feet per second
H42 = temperature
H43 = barometric pressure, in Hg

There are constants/standards like 30, 2800, 460, 59, & 29.92

I have managed to duplicate Sg values using this with those in the Berger site, but have used the total bullet length with no plastic tippy thing (Berger does not make bullets having plastic tips) Apparently, Miller & Courtney collaborated to make modifications to the original Miller twist program to allow for the little plastic tippy thing which commonly measures about .14 inch - the value for this is H39. Upon looking at the equation, the operation, (1 + ((H38-H39)/H36)^2)) allows for the plastic tip. I made the modifications to the procedure and was able to match results from those of both Berger & JBM.

I have found Sg's somewhat under 1.4 to be plenty good as well as those approaching 3.0 for my casual applications. I have my own ideas about bullet blow ups.
The JBM calculator (JBM - calculations - stability) allowing for the plastic tips, matches my Excel stuff.

The Berger site uses altitude. The JBM site uses inches of Hg.
Running the .224, 70 VLD (317,922 RPM, no blow ups yet, 7.7 twist) thru both the Excel & Berger site came up with 2.03 Sg (fully stabilized as per Berger). I used sea level or 0 ft. altitude in the Berger site.

Running the .243, 87 Vmax (king of splat, 255,600 RPM, 10 twist) thru the Excel & JBM site with the same stuff including .16 tippy I came out with 1.78 Sg

As evident, this is an estimator and uses combinations of bullet dimensions, ballistic data (velocity & twist rate) & environmental conditions. The actual mechanics of bullet gyro stuff are not addressed. The math is purely high school level.

Plug this stuff into your Excel.

 

[Mr.Knowit all]

Very hot topic. I said before that most old school thought is to get the min. Twist needed. The more progressive thought is to run a faster twist.
I shoot an 1-8.5t from Bart. in my 300wm. It shoots all those bullets you mentioned well.

 

[fyrewall]

Another method might be to use the Hornady 4DOF procedure for the 225 Hornady ELDM which gives a "Gyro" value of 1.789 @ 2850 fps, with a 9 twist. The velocity at 1760 yards is real close to the speed of sound at an elevation of 0 feet and standard temp & pressure. The Sg aspect is addressed on page 6 of Hornady's tech notes. Hornady recommends a "gyro" value not less than 1.4 and provides an explanation for that. There are no references to BC stuff in the Hornady procedure, other than possibly why Hornady does not use BC's.

The rpm's @ 2850 would be 228,000.

For my simple & casual, .30 shooting I use my 10 twist, .300 Win mag with 200 grain Hornady ELDX bullets @ about 2850 with modest load of H1000. I never worry about bullet blow ups - modest velocity, relatively thick jackets, nice smooth lapped barrel, and 205,200 rpm's (10 twist).

Any 300 grain bullet @ 3300-3400 would beat me up real bad.

 

[fyrewall]

If you run the Berger twist/stability on-site procedure any BC, G1 or G7, may be entered or omitted and the resultant Sg is unchanged. The Sg standards appear to be only a fixed statement. It looks like the Miller program is used - BC's are not used in the Miller calculation. The BC stuff could be regarded as a general measure of the bullet's worth and not needed in the Sg determination. The JBM site allows for plastic tips in Sg determinations.

Sg values obtained with the Berger on-site program using plastic tipped bullets would probably give lower Miller Sg values because the plastic tip is much less dense than bullet jacket and would increase the apparent bullet length. The Miller twist program appears to assume and approximate bullet jacket (copper alloy) and core (lead) have equal densities.

 

[Ballisticboy]

If you run the Berger twist/stability on-site procedure any BC, G1 or G7, may be entered or omitted and the resultant Sg is unchanged. The Sg standards appear to be only a fixed statement. It looks like the Miller program is used - BC's are not used in the Miller calculation. The BC stuff could be regarded as a general measure of the bullet's worth and not needed in the Sg determination. The JBM site allows for plastic tips in Sg determinations.

Sg values obtained with the Berger on-site program using plastic tipped bullets would probably give lower Miller Sg values because the plastic tip is much less dense than bullet jacket and would increase the apparent bullet length. The Miller twist program appears to assume and approximate bullet jacket (copper alloy) and core (lead) have equal densities.

BC makes no contribution to SG at the muzzle of the gun which is entirely dependent on the aerodynamic and inertial moments and the barrel twist rate. BC will only have an effet on the change in SG as the bullet moves down range as BC will affect the change in the ratio between forward velocity and spin rate (fixed by the barrel twist rate at the muzzle).