Old luger
 

Hi I inherited a 1921 luger serial # 1078 form my grandfather. My great grandfather got it while in a weapon gathering detail shortly after WW1. I realized I don't see what caliber it is. Any help would be appreciated.

Welcome to the Luger Forum.

You should post a photos of your luger for best results.
Both sides, top, and under the barrel in particular.

You can look under the barrel if it is a military issued luger for the 8.xx barrel size.
Another trick is to try to insert a #2 pencil into the barrel.
It will fit in a 9mm hole, but not a 7.65mm.

Welcome to the forum. I know you meant to say WW2, since 1921 was after WW1.

A dated 1921 would have been for the army or the police, so pictures are needed.
Full left, full right, full top, and any external markings.