i got a browning abolt and i'm lookin at a nikon buckmaster 4.5-14x40 scope...my question is what height rings should i get? low r medium?

I think the medium will probably be the shortest you can use, but I'd take your gun and scope (or borrow one at the store) and set it and the rings on the gun to see if they'll work.

I would go with the lowest you can get, with the 40mm you might be able to get away with the low rings, I would try and check it out first.