According to Warne the M12 & a 56mm objectine would need high rings (assuming 30mm tube). They have a selector assist that you can enter rifle, base, scope size, etc. (They make the Nikon mounts as well, I believe). (PS - if it's a 34mm tube Nightforce I'm greatly jealous).
http://warnescopemounts.com/scope-mount-selector/
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