I think Gewehr76 is talking about using 9mm bullets in .380ACP cases... the reverse of what was asked. Not sure.
As far as using .380ACP bullets in 9mm cases, you've gotten me curious because I reload 9mm too.
Everything I find says the 9mm Luger and .380ACP bullets have the same .355" diameter. The lighter .380ACP bullets are likely a tad shorter, so you may end up with a very short COAL cartridge (min for 9x19mm is 1.000"). Not sure how a fast, light bullet will stabilize but it's likely possible to check for stability online.
Hornady has load data for 80gr Barnes TAC-XP bullets and 90gr Speer Gold Dot Hollow Point.
So I think that yes, you can use .380ACP bullets in 9mm cases, but you're really in the land of experimental loads. You cannot use the load data for .380ACP because that round is rated at max pressure of 21 500 PSI by SAAMI, not at the 35 000 of the 9mm Luger. So you have to use 9mm load data, extrapolating for a lighter bullet.
Barnes 9mm Luger TAC-XP
Diameter .355
Weight 95
Length 0.597
S.D 0.108
B.C 0.120
Barnes .380ACP TAC-XP
Diameter .355
Weight 80
Length 0.525
S.D 0.091
B.C 0.107
Above all, don't take my advice as holy writ. Research!
