As for dimensions of your Performance center gun there is only one way to know what internal dimensions are, and that is to slug the gun. As for doing improvement work, lapping the barrel, using the LBT bore lap kit, will almost certainly improve accuracy and overall performance. If there is any roughness, constrictions or other changes in internal barrel dimensions, lapping will correct them and make the barrel shoot with optimum accuracy.
When you purchase powder again I recommend 296 or H110, but burn the 2400 you have first. If the bore and other internal dimensions are good, you should be able to just pick any load from a manual which promises the velocity range you want and get excellent accuracy. The aimpoint sights are real nice for hunting, but I have never been able to shoot tight groups with them, due to the large aim point and the fact that the dot obscures the mark on the target. In other words, in my opinion, you may be holding as well as possible with the aimpoint. I'm not a good judge on those sights though as I never used mine very much. When I was shooting one day soon after I got it, the battery failed to keep contact after a couple of shots, and I had no aim point. I took the sight off and never messed with one again. Switched to a 1X scope which isn't likely to lose it's cross hairs. Had to do something as my eyes had failed enough at that time of my life that I could no longer use iron sights effectively enough to suit me.
Both 260 and 280 gr are good choices, but accuracy will probably be slightly better with the 280 gr, due to longer bearing length.
I favor 296 and H110 ( which are the same powder) because it gives more uniform velocities, and much longer gun life than 2400, with particularly the throat erosion issue being tamed by the ball powder. When using stout loads, don't even consider 2400 or other stick powder. Use the ball powders.
