De41Mag. there were several shooters it sounds that turned in 44 targets in the match, I hope you wait to see how everyone did. I shot a cast bullet load for the match and I bet that at 100 yards it won't shoot any better than yours. Mine will put four shots practically on top of one another at 50 yards and always lets shot 2 or 3, usually 3 stray down and away from the group, I have never tried to solve this, I got the rifle mainly for a plinker and short range woods rifle. In reality, I bet that 2-3" groups from most 44 rifles with cast is about the norm, I cannot speak to the velocity other brand rifles may drice them at.
The last jacketed load I tried was Hornadys 300 XTP, it shot well enough, but not as well as the Speer 270. I bought a different primer, Federals Large Pistol Magnum. I had been using Winchesters Large Pistol for standard or magnum loads. That primer seemed to be very consistent and I got very good groups from it. I have ten left. I am going to load 10 rounds of the 270 GD, 5 using the Win primer and 5 using the Federal, and see what happens, I suspect at this moment that the Federal doesn't shoot quite as tight.
I'm sorry to hear that you are having a hard time with your rifle. I wish I could buy it but am not in a position to do so. That the rifle shoots poorly with both barrels suggests to me that there is a fit up problem of some kind going on.
I know what it is like though to have had enough......