Are piston and end cap included?

Pistons and end caps are chosen to match the cartridge size. A piston that jams or lets air pass is the most common source of error in the whole set-up.

What are opaque cartridges for?

Light curing media react inside a transparent container. For those, cartridges that block light are used.

Which cartridge size is the right one?

Size is a process decision, not a price question. A larger container cuts the number of changeovers per shift but extends the time the medium spends in the open system. With a limited pot life the smaller cartridge is the right one.

Why does the shot size scatter despite identical settings?

The most common cause is trapped air in the medium. Trapped air is compressible and cushions every pressure pulse. The second cause is a leaking piston: if air passes it, pressure builds slowly and the first shot after a pause comes out too small.