Huvius- The back is solid. Rob drilled hole in the front to within
1/4" of base. Hole was 7/16" then he drilled 1/2} hole in
3/4" and put in insert, against step. Idea of aluminum and hollow
inside is to get front heavy projectile stable at slow twists or
in smooth barrels.

Daryl- Those are brass bullets for .620 cal rifles, 600NE,
and Rob'd 600OK. They work in a 20ga with .620 rifled barrel. Ed