....A hatton round may be a slug but it is not by any means a 'solid slug'....
So is it liquid ?? So....ergo...it must be solid.
A solid slug is a single lead or metal projectile
Doesn't have to be...... There are many different materials used for solid shot - depends on what effect you want or what you're attacking.
I tell you what R44-Pilot....here's a test for you.... Try firing a Hatton round down a fully choked shotgun barrel and I wager the results will tell you if it's solid or not
What do I know anyway.....
Ammunition Technical Officer - Wikipedia, the free encyclopedia 
LMAO