WHAT!! No no no. Use the gauge! Never go over the specified pressure. Some cylinders will permanently deform at the threads at 300 as a safety feature.
If the cylinder has its own gauge it will be damaged too.
If the OP is wanting to equalise an overfilled tank with a 200 bar cylinder with no gauge then (s)he needs to change fill method to the recommended one.
I was going to jump all over that post until I saw this response on the next page.
That wrongheaded thinking, if the tank is tested to 5/3 of working pressure why can't I use my 4000psi tank at 5000psi, got a few people injured in Paintball.
Fill cylinders to their working pressure only and never beyond. If you're regularly over pressurising the tank (even briefly) or worse, aren't sure of the tank pressure, you're doing it wrong.
If you want to make sure every one is safe, get a regulated fill station.