Thanks, folks, for your comments and advice.
CoyoteJoe: No, it's not a two-groove. The rifling is worn but the bore is shiny.
broom_jm: That's something seriously to worry about. There's a thread somewhere on the forum that I found where just this happened. Fortunately the chamber was a bit oversize rather than undersize, so the brass got blown out instead of the receiver.
zb338: That's correct, though there is a backup of sorts in the bolt handle and what might be a safety lug near the back of the bolt. Following the Spanish-American War the army decided to heat up the 30-40 cartridge from 2000 fps to 2200 to compete with the Mausers the Spanish were using, with the result that that single lug started cracking. The hotter loads were speedily abandoned and the 30-06/03 Springfield developed instead. One of the reasons the Krag action is so smooth is because the steel is rather hard and therefore brittle. It's best to load conservatively.
True story: A fellow had just taken delivery on a pristine 30-40 Krag rifle, on our range, from the dealer, and occupied a bench to shoot it. Why he did this I haven't a clue, but he'd brought some original (as in "antique," doubtless mercuric-primed) military ammunition to shoot in it. The first round went "pfhhhht." Range officers were literally in the air trying to get to him and stop him, but he jacked the next round into the chamber before they arrived and blew six inches off the end of his barrel. I can't tell you why the receiver didn't shatter; I don't know. No more pristine Krag.
I still use this example in my safety briefings.
O'Connersun: That's a good suggestion, but it isn't mine - yet.
Thanks again, folks!