60gr TMK accuracy/RPM's

[jelenko]

Has anyone found that spinning these over 275K significantly opens up the group size?

I'm currently trying to use these in a 1:7.7 26" barrel with 8208. The only fairly accurate node [~ 1MOA] I've found is 23.4gr [MV 2920].

At 2920 MV, RPM is 273K. As the MV goes up, the groups open up.

I know 7.7 is too fast for this bullet, but just want to see if others are finding a kind of max RPM for accuracy.

Thanks

 

[chromatica]

From the parameters you provided, the Miller Stability index is 2.3. That is considered over-stabilized. When a bullet is overstabilized, it amplifies any dynamic imperfections of the bullet. One of the effects is a phenomenon called lateral throw-off. Lateral throw-off occurs when a bullet transitions from spinning about the bore axis to spinning about its center of gravity, right as it leaves the muzzle. When this happens, the effect is a small angular displacement which causes your groups to open up.

If you changed the twist rate to 9, the stability index would drop to 1.6.

 

[xswanted]

I don’t have a fancy scientific answer but I’m shooting the 95 6mm TMK from a 7 twist at 3160 with exceptional accuracy at all distances I’ve shot the combo to up to 1050 yards.

I think a guy can certainly overspin a bullet but it’s possible you’re just in a “node” that doesn’t shoot very well.

 

[jelenko]

Yeah. I was just wondering if the 60 TMK didn't do as well as other Sierra bullets when over stabilized.

That does seem unlikely; just trying to understand what I need to do.

I have shot 4 shot groups from 23.0 to 25.3 in 0.2gr increments. But, thinking back, I might have missed some steps - they were shot at different times. Maybe redo the ladder test.

 

[venatic]

They shoot great in my 1/10 twist at 3125fps. I am using 24.0 gr of N133

 

[jelenko]

Thanks to everyone who posted!

I found the problem. A component was having a problem.
This component has given me problems several times over the years.
I've tried getting rid of the component, but it can't get it to work.

 

[Mad_Charlie]

Could you elaborate a little?

 

[AckleymanII]

I shoot them in a 22/204 in a Shilen 9twist,26" and with cfe, there are nodes at 3150, 3250, and right at 3400....no loose primer pockets with Win brass. Reamer has .020 freebore, JGS reamer, no turn neck.

 

[jelenko]

Could you elaborate a little?

Sorry, trying to be subtle. That hard to train component is me.