Welcome to the forum... and to the wonderful world of Krags.  Be careful, they're addicting!

Culpepper and Butlersrangers are right on.  As Culpepper said, drop a cleaning rod down the barrel against the closed bolt and measure it.  I agree that it's most likely a cut down rifle barrel used to replace a damaged/rusted/pitted/shot out barrel.  Krag barrels, both carbine and rifle are available from Criterion these days... but this was not always the case, so replacing a carbine barrel with a cut-down rifle barrel would seem like an attractive idea to replace a damaged barrel back in the day.  Just my $0.02