I'm not sure what the stock size of the Etha barrel is, however to improve accuracy I echo what Alexd says above.
What is more, you want to try matching the size of paint to the bore of the barrel.
Better quality paint is generally of a smaller bore, and so requires a smaller bore barrel.
I'm not sure how much shaft-4 boost barrel kits go for nowadays, but £100 should fetch you one, with a range of bore sizes.
The general rule of thumb when matching paint to the barrel, is if you drop it through and it stops partway down but can be blown out the other end, then it's generally a good fit.