This projectile looks like a 20x99R Russian ShVAK maybe a practice ?
It is steel with a plain base with the edge turned. Nose does not unscrew.
Somebody may have cleaned the groove or it is remains of solder or sealant?
Body has a slash 41 stamp and maybe another letter to the left I cannot ID.
Length is 55.1mm.
Body diameters are 19.9mm above the drive band. 19.6mm below it.
Projectile weight is 78 grams or 2.7oz.
I am keen to know the type, case, country of manufacture if possible.