Extra bonus update...
Took a handful of spent cartridges to the smith today, along with the rifle. It seems the imprints in the case and primer are a function of the way the lower claw of the extraction mechanism beds into the bolt face next to the firing pin when the action closes. They are exactly identical across all the fired rounds, and he didn't think that there was anything to be concerned about, but he did say I was right to bring them in and check.
So thanks again O_P for the heads-up. Well spotted, sir.
With my mind at ease, I took another box of ammo back to the range in back of the shop and sent it heading toward some paper. Except this time I shot standing on my own two feet, instead of sitting at a bench...
What a difference! The recoil when standing is enormously more tolerable. I feel like I could shoot it all day that way, whereas on the bench it was getting sore after ten or fifteen rounds.
Although I was consciously trying to let the trigger surprise me and avoid flinching, honestly, standing up it just wasn't a problem. This range is only 25 yards, but my groups were down to 1.5 or 2 inches dead center on the bull - much more like what I am normally capable of with iron sights from standing. Now I just need to work on getting the same discipline sitting down. Hopefully being a bit more confident that I can shoot it comfortably will help when I'm next at a bench.
I am all the more pleased with this thing.
Next step - water jugs. Some of the chaps from work have a trip to an outdoor rod and gun club around here happening the weekend after next, so I'll give the place a call and see what I can arrange.