A barrel roll is a positive G maneuver. A slow roll is -1 G while inverted. It can be done in any attitude from horizontal to vertical.
Come to Reno for the Reno Air Races this week with us and you will see it all. My friend flies Voodoo again this year and has won the last few years in a row in a P51. His Dad gave me a couple aerobatic flights in another P51 years ago.