[MadMax]

Looks like a gas flow restriction based timer similar to what's in a Deepfire grenade. Allow the valve stem to rise and gas flows through an adjustable restriction until a pressure difference across a part decays and the valve body pops open releasing gas.

The pneumatics are inside the thick walled body (~6mm thick) which hold gas and perform delay function.

A mesh inside the body prevents bbs from falling into the body and rattling around.

A plastic sleeve with holes (a little less than 6mm) around the outside retains bbs so they don't fall out of the body (actually an array of 6mm long barrel).

A gap around the valve body and mesh acts as a pressure manifold which applies equal pressure to the backs of the pellets.

I'm guessing pellets are pushed past the sleeve individually (ugh...) so they seat inside the thick walled body between the mesh and the undersized sleeve holes.