Zeroing a Heym 270 win. It has an old redfield scope. With Caldwell deadsled, at 25 yes " right on". , at 50 yards 3/4 inch high, at 100yards not on the paper (15 x15 inch). I'm thinking bad scope?

Start with the bases/mounts and the rings the scope is mounted on and go from there. Has the gun been shot before with another scope on it to rule out its the rifle itself, rather than anything to do with the scope and mounts?

as topgun said, start with the bases and rings. if they are all tight id check the scope for parralex. to do this, set the rifle on sandbags or in a solid rest. without touching the gun look through the scope at a target. slowly move your head up and down and side to side. if the crosshairs seem to be moving on the target your scope might be done. time to replace with a new one.