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Larry Gibson’s post from “other” board

RPM Threshold barrel twist/velocity chart RPM threshold twist/velocity chart

I’m posting this at request for an easy reference to see the velocity range where the RPM threshold will most likely be found based on the twist of the barrel.

The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent. It is best noted when working up a load as velocity increases flyers begin to happen. Then as velocity is further increased the total group size increases sometimes to the point some bullets fly so far off they miss the target. A further indication the cast bullets at or over the RPM threshold is (or some of them in a load that is on the edge of the RPM threshold) the non linear dispersion of the group size as range increases.

Let us keep in mind the RPM threshold most often falls in the 120,000 to 140,000 RPM range with regular lube groove cast bullets. Exactly where the RPM threshold will be in fps depends on numerous factors; alloy, bullet design, fit, sizing, lube, GC’d and seated square, powder burning rate and the length of the barrel, etc. The RPM threshold may be lower than 120,000 RPM by careless casting and loading techniques or when using very soft alloys with very fast burning powders. Conversely, the RPM threshold can be above 140,000 by careful casting and bullet selection and preparation along with careful accuracy enhancing loading techniques, especially those for cast bullets at high velocity such as using slow burning powders that ignite easily and burn efficiently at lower pressures. The trick is to get the cast bullet to exit the muzzle as balanced as possible with as little deformation to it during accelleration. The more balanced the bullet is and the closer the axis of rotation coincides with the center of mass on exit from the muzzle and during flight the more accurate the bullet will be and thus, the higher the RPM threshold will be.

The RPM threshold is not a set “limit” of RPM or velocity. Best accuracy will be just under the RPM threshold or lower. Useable accuracy can be had above the RPM threshold if the ranges are not long and the accuracy requirement is not small. Keeping .223 cast bullets on a silhouette target out to 200 yards for example or keeping hunting cast bullet accuracy at say 4 moa if the max range to be used is 50 – 100 yards.
Again; the RPM threshold will generally be found between 120,000 to 140,000 RPM with regular commercial cast bullet designs and loading techniques most cast bullet shooters use.

In the chart below I’ve computed the fps for various common barrel twists for 120,000 and 140,000 RPM. For other twists in between anyone shouldn’t find it too difficult to interpolate. These fps figures should give you an idea in what fps range your loads, as you work them up, will probably bump into the RPM threshold and when accuracy will probably begin to deteriorate. Some pundates will crticise this chart saying they, or someone else, gets accuracy above the figures in the chart. For those who understand how to push the RPM threshold up with higher velocity cast bullet loads that can indeed be the case. However, as mentioned, the chart is for the majority of cast bullet shooters who do not care to push the RPM threshold up but simply want to understand where and why accuracy will probably deteriorate with their regular cast bullet loads. This chart was done for them.

RPM……….120,000……….140,000

Twist……….FPS…………..FPS

7″…………1166…………..1361

8″………….1333…………..1555

9″………….1500…………..1750

10″…………1666…………..1944

11″…………1833…………..2139

12″…………2000………….2333

14″…………2333………….2722

16″…………2666………….3111

18″………….3000…………3500

Larry Gibson

If I’m only looking at 2700ish fps w/ 180 grains and
the long 1.4″ 300 grain is only going 2000 fps
which gives it SG=1.87 and at 1080 fps it’s 1.5 SG

wouldn’t the 1:14 twist maybe be better?

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