Though it's not an H&R Handi rifle, you might want to check out all the FAQs in the centerfire forum over there. I am sure most all of the suggestions there will apply in your case.. forearm pressure, bore smoothness, latch isues, etc will all help in your case.
Those WWB 45gr JHP's shoot excellent in our Handi rifles, wish I could find more around these parts.. but then again Handloading is such a freedom
My suggestions , if I may, a GOOD barrel scrub with Flitz and time to let the slim barrel cool enough between shots.
-Aaron