You need a dedicated adapter to fit a 300cc bottle and I only know one person who makes them. (I have a spare as I bought two.)
You also need a complete replacement shroud as the Best Fittings 300cc bottle is a sufficiently larger diameter than the standard one that it won't fit with the standard shroud.
The photo shows how it looks. The top gun isn't mine and has a 300cc bottle and 30mm shroud, the lower one is mine with standard bottle and 30mm shroud.