I have used a lot of backer rod in .450BPE, .500BPE, and .577BPE. I am yet to see any adverse effect.

I have also used Dacron, foam rubber, and felt wads. My preference is backer rod.

One caveat: backer rod doesn't work well in bottleneck cartridges. In my .450/400 3-1/4" BPE I use either foam rubber wads or Dacron.

One last note. Sherman Bell contacted me earlier this year urging reconsideration of the use of backer rod. He told me he personally knows of a rifle whose chambers were ringed, and he thinks backer rod is the culprit. I don't have any more detail than that. It always seems there are contrary experiences or anecdotes.

I wouldn't hesitate to shoot that H&H with proper NfB loads. Let us know how she does.

Curl