I'm sorry...I must be missing something. The written story states... "Bastard! Champlin yelled. He fired the CARBINE until the clip was empty"... So it started off as a carbine.
Here's an addendum from the book that I think this came from, actually quoted as a carbine... So, buy the rifle not the story? The Battle of Bataan: A Complete History, 2d ed. By Donald J. Young, page 72.