How fast a twist rate is to fast

[fyrewall]

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).

What I see:

Looking at the Hornady 4DOF process I see that values in the "GYRO" column increase ("change in Sg") as the bullet moves down range. Using the .30 225 ELDM bullet @ 2850 the initial GYRO value is 1.7 something and the GYRO value @ 1760 is over 5 something.

What I also see is the Miller Sg estimator process accepts data like: bullet weight, bullet diameter, bullet length, twist rate, temperature, and altitude/pressure and produces a Sg number - no "aerodynamic and inertial moments". No bullet form - it can be a chopped off slug or a real pointy projectile. The Berger and JBM sites produce equal Sg numbers using the same data and BC's are not included in the process but recommendations of minimum Sg numbers are. My simple Excel excursion does the same.

The Hornady 4DOF documentation does compare ballistic data obtained using G7 BC's with those obtained with the 4DOF process. Certain other bullet makes are available for 4DOF use.

Whenever I plan a re-barrel job I look at the 4DOF process as a guide in bullet & twist rate selection.

 

[FrankG]

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.

That’s why I don’t like tight bore/tight groove barrels for the most part. Your amplifying or making problems out of something that might not be there.

 

[canderson]

FYI. Been running 215’s out of 8 twist 300 NM. No failures yet. Velocity approx 3100

 

[Ballisticboy]

What I also see is the Miller Sg estimator process accepts data like: bullet weight, bullet diameter, bullet length, twist rate, temperature, and altitude/pressure and produces a Sg number - no "aerodynamic and inertial moments". No bullet form - it can be a chopped off slug or a real pointy projectile. The Berger and JBM sites produce equal Sg numbers using the same data and BC's are not included in the process but recommendations of minimum Sg numbers are. My simple Excel excursion does the same.

The Miller estimator and others like it are just simple empirical curve fits of common bullets relying on a uniformity of design and construction, not accurate descriptions of the important variables, intended for users who have no access to the real variables. If they include "aerodynamic and inertial moments" or not is irrelevant to the definition of SG since they are not supposed to be scientific descriptions of the SG variable merely attempts to produce an approximate value based on easily measured variables. The true SG equation contains both aerodynamic and inertial moments and is the only truly accurate equation based on aeroballistic fact for all projectile designs.

 

[dcali]

What I see:

Looking at the Hornady 4DOF process I see that values in the "GYRO" column increase ("change in Sg") as the bullet moves down range. Using the .30 225 ELDM bullet @ 2850 the initial GYRO value is 1.7 something and the GYRO value @ 1760 is over 5 something.

What I also see is the Miller Sg estimator process accepts data like: bullet weight, bullet diameter, bullet length, twist rate, temperature, and altitude/pressure and produces a Sg number - no "aerodynamic and inertial moments". No bullet form - it can be a chopped off slug or a real pointy projectile. The Berger and JBM sites produce equal Sg numbers using the same data and BC's are not included in the process but recommendations of minimum Sg numbers are. My simple Excel excursion does the same.

The Hornady 4DOF documentation does compare ballistic data obtained using G7 BC's with those obtained with the 4DOF process. Certain other bullet makes are available for 4DOF use.

Whenever I plan a re-barrel job I look at the 4DOF process as a guide in bullet & twist rate selection.

I think you'll find that 4DOF and Miller Sg's will be very similar for normal bullets. I've done the full calculations (the ones 4DOF uses) with values that have been calculated by various methods, and they almost always wind up being close enough to each other to just call it the same.

As Ballisticboy says above, Miller assumes normal shaped bullets. It will not work for a wadcutter, for example. 4DOF *should* be better, given good inputs. The only fly in the ointment is that the coefficients required to get a good stability number are not easy to get, and last I heard, were calculated with PRODAS. That should be fine for our purposes, but who knows.

 

[fyrewall]

My read on this stuff:

There is no direct numeric relationship/connection between the Sg values determined by the Miller method and stability assessments in the Berger calculations. The Miller method pumps out a Sg number that assumes the bullet is solid copper or copper jacket/lead core - plugging in data for a round nose bullet having the same weight, diameter, and other equal data will produce a higher Sg because the round nose bullet is shorter than a pointy bullet of the same weight, bore size and velocity.

An apparent comparison with some standard 30 caliber bullet @ 2800 fps is made - ((30*bullet weight)/(twist/bore diameter))^2 and then velocity/2800 - "simple empirical curve fits of common bullets".

The Miller estimator will produce a Sg number of 3.05 for an impractical combo (real short range) - a .224 jacketed square nose (wadcutter?) of 53 grains, .5 inches long @ 4000 fps, 14 twist & standard temps & pressure.

The starting Sg values using the 4DOF (0 range) are lower than Miller Sg's - like 1.4 compared to 1.7. These are for the .224 75 ELDM @ 3350 fps.

In any case when I walk out of my LGS with a heavy box of bullets that I have selected on the basis of either Miller or 4DOF I am confident that they won't go sideways. The 4DOF produces more numbers that appear to be real good for my casual shooting. I struggle to remember & grasp "inertial moments" stuff from long past physics 201.

Should any member be interested in probing my Excel Miller spread sheet, send me a PM & I will send it via Email. It is real handy to see comparative stuff on the same page for a variety of bullets before producing a credit card.

 

[mikecockcroft]

my head is spinning at 310,000 rpm's after reading this, hope it doesn't blow up