I believe that my 160-600 c has a stiff focus ring. I compared it to my smaller Sigma lenses and they rotate freely.
I have also tried a friends tamron 150-600 and his turns as freely as my smaller Sigma lenses.
C.R.Kennedy said it was normal, but to get the focus ring to move, the lens is lifted/moved before the ring rotates.
So the initial force to move the focus ring will have to be heavier than the lens.
Anybody on the forum with a Sigma 150-600 C, can you test your focus rings for easy movement please and reply.