I use leupold rings and bases and have never had a problem with them coming loose or the zero moving.As far as sighting in your gun I would sight it in at 200 yards then shoot it at 100 yards and 300 yards to verify the point of impact.Using a 50 yard sight in without actually shooting at 100,200 and 300 yards could leave a great deal of error at longer distances.As well if you intend to be proficient at 300 yards you will need to practise on a regular basis.Far too many people sight in at short distances without ever practising at the distances that they intend to shoot game at and the result is often missed or worse yet wounded animals.