I made a quick research and this is the only thing i found about the "tromboncino Mod. 28"
http://www.il91.it/Sito/Fucili1891/91-28tromboncino/Dettagli28Tromb/TromboncinoM28.htm
I read the article, IMO it was just fragmentation and it was not used as anti-tank weapon:
(...)launch, until to 200 meters,infantry bombs against targets defilades to you (google translation)