My arrows are rated with spine stiffness of 0.340 and 9.5 grains per inch. Do you think i might be better off going to the ST AXIS 500's that are only 8.1 grains per inch and a spine stiffness of 0.500? I just dont want to under spine my arrows....
You've got it backwards, it seems. Going to .500" deflection is going to a weaker spine than 340's and would be underspined for a bow that needs a shaft that deflects.340".
An arrow that is so stiff it cannot be deflected by the spine tester would have a deflection of zero. On the other hand, an arrow that deflects a full inch in the spine tester would be very weak.
The lower the deflection number, the stiffer the spine. The hgher the deflection, the weaker the spine.