If we're willing to consider the assault rifle in its applied role, and not just by an arbitrary definition, then I propose that the first "assault rifle" in application was the 1860 Henry and the first intermediate cartidge was the 44 Henry Rimfire.
The Henry rifle first saw combat near the end of the US Civil War but rapidly cut a bloody swath via the Indian Wars and throughout the "Old West".
Please consider supporting the InRangeTV project via Patreon: https://www.patreon.com/InRangeTV
We have T-shirts! Hooray! https://shop.bbtv.com/collections/forgotten-weapons?sort_by=created-descending