Interesting question, however I believe it may be sinceit is in between the S300 and X100 shaft flex.

6.0 has always been stiff in project X and 5.5 was between regular and stiff.

www.pxshaft.com/.../

We sat down with our shaft engineer and guru Don and here's what he had to say.
There are many more sub flexes with Project X, so the standard flex designations for irons has not been as clear. When we chose 5.5 for stock in our original AP2 ten years ago, we didn't see that club as a Tour played model. We expected it to be played by the better amateur so we tested both 5.5 and 6.0 with our test groups and the 5.5 won. As we know now, since they were introduced, the AP2's has been the number one model on Tour and that continues today.
Our new family of 718 irons offers distance and forgiveness models, such as the AP1, AP3 and T-MB and players looking at the Tour models (in general) are on the stronger, faster end of the spectrum.
With the introduction of additional models in the PX family, such as PXi and LZ, the 6.0 has become more accepted as the standard "stiff" flex equivalent and we have gone with the 6.0 to be more consistent with current market expectations. Hope that helps!

What is your opinion on finding the spline on an iron shaft and should it be aligned with the face of the club? Does a rifle shaft have spline?

Sorry to disappoint but 6.0 has always been stiff

I have the Hzrdus smoke yellow in my driver and smoke black in my fairway.

I love the Hzrdus line they look unreal and feel incredible!

Hi,

I think if you compare the dynamic gold s300 to a project x 6.0
The dynamic gold s300 is at 5.8 if I remembered correctly.
So 5.5 is a bit softer than s300 and 6.0 is a bit stiffer.

I would say 5.0 is regular, 5.5 is firm and 6.0 is the stiff of Projectx.

Hope this will help you,