is your problem front to back distance too short for the length of the scope or the 50mm dia of the scope sitting on the barrel? depending on the gun,you may need high mount rings to allow clearance between barrel and scope.if front to back distance is the problem,you will probably need different bases that have adjustments forwards or back to fit the length of your scope.all advice given previously should help you zero in on a solution.WORD OF CAUTION make sure you can see daylight between barrel and scope at front end!