Have a H&R rebored to 357 Max. and am trying to shoot 180gr cast PC bullets at .358. Using Lil Gun. at 19 gr. I get a 2.5" group at 20gr it opens up to 5" at 50 yrds off rest. Am I trying to push the Polymer coated bullets to fast ? Note: Barrel slugged and I measure it at about .357 and chamber is tight an will not take a .359 dia bullet.