Take the barrel out of the stock, and check for a date stamp. With no date stamp and the old proof mark with the proof weights, the .318 I barrel is a distinct possibility. If there's a post-1920 date stamp I'd guess .323 IS bullet.
I used to love being able to hit hard at 1000 yards. As I get older I find hitting a mini ram at 200 yards with the 22 oddly more satisfying.