Can’t properly adjust bison 6 jaw set tru

Are you taking your reading off the chuck body? If so their might be a thou or two runout depending on chuck quality did you say what brand it is? Reread your post a bison should be pretty good. You might want to separate the chuck from the mount and measure to see what range the chuck is capable of. Might have to take off alittle more of the flange like others have stated?
 
There is a good possibility that if this chuck is a newly acquired used piece, that the "adjustable back plate" is not the correct one for that chuck which may be corrected by turning down the center Register like parshal did.
 
I have the same chuck in 6.3". It's a little bit more trickier to adjust than say a 4-jaw. The chuck has multiple pinions so the adjustment screws aren't spaced exactly 90°. You have to loosen one of the other adjustment screws that's closer to the one you are adjusting. Once you get used to it it'll be easy.

My 6.3" Bison 3-jaw is the same way. My Bison 5" 6-jaw only had one pinion so it did have the adjustment screws evenly spaced 90°.
 
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