Inner barrels = increased accuracy + a slight FPS increase
They are one of the first upgrades someone dose because there simple and do make a deference
The most popular type of barrel is a 6.03 (this is the diameter of the barrel) but you can go all the way down to a DBC 6.01mm tight bore (this is not really recommended because ideally they would be used in a sniper and you will spend half your time cleaning it to prevent blockages)
Just for some info a standard inner barrel is about 6.05-6.08
Range wise your looking FPS but this is a dodgy way of increasing range because you need to make sure that your well within your airsoft sites FPS limit or you may be rejected!!
The main way to upgrade the FPS is to replace the main spring with a more powerful one
You might want to get a shop/experienced player to do this as it requires stripping the gun