matching cylinder to barrel

how do u match a cylinder to a barrel?
The barreL In my gun is about 430mm,and the gearbox is just the stock G&G blowback.
Could I get better compression/efficiency if I disabled the blow back and changed the cylinder?

I think what you are referring to is taking the volume of the cylinder as compared to the volume of the cylinder. volume= (pi) x r^2 x h as both the inner barrel and the cylinder are both cylinders. So compare the volume of the 2. r is the radius and h is the height of length of the cylinder. If your cylinder is ported you need to take the measurement of h after the ports. There is a lot of debate on what advantages there are to having different ratios, but that's a different topic.