what kind of mushroom is that?!
It looks like someone trimmed a .303 case back to the shoulder and loaded a .458 pill into it…
that sounds cool, but… no, way smaller.
no takers? meh.
320/230
No, but we are sort of in the same domain if you will… All sort of around there. VERY close.
Let me know when you give up, most of you know what it is, shot it, held it, etc. You are just not used to seeing it like that 
Clue: for sporting and civilian use cartridges were sold with lead bullets. For military use, FMJ ammo was used.
Wait, it’s not that trapdoor black powder round is it? The one from last plinkfest?
.577 Snider?
It is not, but if only there was a smaller similar round used for training cadets?
22lr.
Is it 310 cadet?
Is it a rimfire?
Yes, it’s rimfire.
Some sort of 17 derivatives?
Damn, you guys are good at this. It is related to 17 in some way.
Is it a 17hmr?
Too short.
17 hm2