This ATF webpage quotes the Gun Control Act’s definition of “rifle” and provides illustrations of various rifles:
In case anyone wants to see the laws and regulations related to the definition of “rifle” on the ATF page linked above, that page links in turn to the statute itself and to the accompanying regulations, which seem to contain the same definition as that of the ATF page linked above. Below I’ve posted pertinent parts of the pages that the ATF page links to (although the find function might come in handy on the pages linked below):
This is a link to a page containing the definition of “rifle” in the pertinent statute.
This is a link to a page containing the definition of “rifle” in the pertinent regulation.