I have a impact 737 with a 26in bartlein 1-7.5 twist. The first 600 rounds shoot great at 0.3 moa at 2940 fps with SD's around 4. After that I started getting big velocity jumps off 35 to 50 fps after every 80 to 100 rounds and having to rezero every other week. Now my I am at 3080 fps with SD's of 11 on the same load which is 2 grains over minimum. It is still shooting 0.5 moa groups. If try to change anything to the load I get lots of pressure. I double checked all my equipment and measurements. I also tried different primers, powder, and brass with the same result. I don't shoot the barrel very hard and clean it well every 150 - 200 rounds. I now have almost 1600 rounds on the barrel. I am wondering if I should pull it off and get a new one. My buddy has the exact same spec barrel and his shoots lights out with any load he makes. I am wondering if I did something to the chamber.
