Welcome aboard!!! If the frame is an SB1 which originally came with the Huntsman, it should not be used with a high pressure barrel like the .243. If the frame and barrel were factory fitted, the last 3 digits of the frame serial number will be marked on the left side of the barrel underlug. If they match, I'd contact H&R and see if you can't get it fixed. What color is the frame? If it's color case hardened, kinda of speckled, it's probably an SB1 frame.
Check it out,
Tim