I have a pardner handi rifle in .223 with a heavy bull barrel that i hunt with. never had any issues out of it. However I am now trying to push how far I can shoot with it. It will shoot dead on for a few shots, but once the barrel gets even slightly warm to the touch (3-5 shots) the accuracy seems to vanish. I mean from sub MOA to 4 MOA. Any idea what could be wrong?