Wow... I didn't expect this many responses. But this is a fun discussion.
The "penk" I mentioned in the original post is the sound produced by my Mosin Nagant, on a steel plate maybe 1.5" thick, suspended on a rocker arm, at 300 meters. I think I remember it because it was so gratifying, shooting my 80 year-old rifle with ammunition older than I am, at a 10" target 300 meters away, with just the irons; it was my last shot of the day (I figured I'd pack it in after that, since it seemed a good way to end the session, and it was the last round in the magazine anyway).
I'm still relatively new to shooting, so this was the longest shot I've ever made. Hoping to make many more like it.