I've got a TC venture in 243 Win that is giving me fits with ES. Ive ran different powder/bullet/brass/primer combos but this particular rifle has ES of around 150fps(sometimes higher) regardless. Its shoots 1/2moa groups @100y but of course with those ES it would probably string at longer ranges. It will normally start out high and the velocity will decrease with every shot, not always but common. Same powder scale and same chronograph used with diffrent rifles that have low ES. Is there something with the chamber/barrel that could be causing this?