What you have is a casing from a 77mm German field artillery piece, possibly like this: Some of the stamps:
- St: Strengthened case
- HL: Haniel Luege Düsseldorf (brass factory)
- 25: Inspection Mark
not sure yet on the G(may indicate maker Geschossfabrik), and 36 may be lot number. MarzMärz 1917, of course, would be the date.
some sites to look: