Very stange projectile

[APFSDST]

Good day all!

I have a very strange projectile from WW2(?).
It seem like fired from a less caliber gun than the projectile's original caliber!
If possible, have two questions:
1.: What is the original type of the projectile?
2.: What gun fired from?
Some information: the projectile now about 214mm long and about 77,4mm wide. More measurements on the pics!
The rifling's number is 24.

 

[APFSDST]

More pictures:

 

[MOLTKE]

24 rifling's, could it be for the Russian 7,62cm M1909 ?

 

[hasag]

It seems like a gauge fired by a lower caliber weapon and left the mark of the grooves in the projectile.