To mount the Burris Fullfield II 6.5-20x50 mm Rifle Scope to a Ruger American Hunter in 6.5 Creedmoor I recommend using the 1" diameter medium or high rings at the link below.
https://www.opticsplanet.com/burris-xtreme-tactical-1-two-rings.html?_iv_ring-height=high,medium
It is second focal plane.
Much of this will be reliant on the shooter. 20x may be sufficient magnification for some, but an optic with greater magnification may be preferable.
For a flat top receiver, such as that of an AR, would require Extra High rings.
The Burris Fullfield II 6.5-20x50 mm Rifle Scope can be used with a Browning XBlolt in 300 Win Mag.
A first focal plane reticle enlarges and shrinks as you adjust the magnification while the second focal plane remains the same size. For more information, email us at Techsales@opticsplanet.com.
You would want medium height rings
All of the Fullfield II scopes use the capped hunter turrets. If you jump up to the Fullfield IV or Signature HD, they have external turrets. The Tactical models do as well.
50mm objective scopes should use high rings. So for this model you can use any 1" diameter, high rings.
This would be the sunshade for this optic.
MPN: 626032