After understanding the basics of how a gearbox works we can now go into some of the more complicated stuff such as increasing the accuracy of your gun.
One very important thing you want in an airsoft gun is accuracy. Before I get into how to increase your accuracy there is one thing I need to clear up. Accuracy is consistency, meaning you want your bb to travel where you are expecting it to go. The gun is always accurate in the sense that the bb will travel where it's going to travel, but you want to know where it will travel and you want it to go there every time.
One way of doing this is upgrading the quality of the inner barrel as well as the diameter of it. The tighter the inner barrel is the less room it has to veer off in a different direction. The quality of the inner barrel is key for overtime ware and are keeping it as seamless as possible.
Another way to increase accuracy is to upgrade the bucking (rubber piece on the barrel for hop up) and here you have a few options. You can simply go with a higher quality bucking or you can go with different styles. The typical style you will find in most guns is around oval being pushed down upon the bb from the top. This works but is not the most effective as if it is even 2mm off center your bb will veer off in a different direction then desired.
Another form of hopup is called the flat hop. This is essentially a flat nub being applied to the bucking to give it an even flatness over the bb so that there is no one point that has more pressure on the bb.
Then there is the r-hop, r-hop or round hop is a piece of rubber that is fitted over the window in the barrel and seamless with the outside of it as well as with the inside. It's essentially filling in the window as if it was never there but its rubber and not metal. This applies a more even spread of hopup going down nearly half way and has contact on all parts of the upper half of the bb. This is more consistent and so creates better accuracy.
The downside to an r-hop is it has to be very very precise, meaning that each rubber piece is fitted to the inner barrel and cannot be just bought and slapped on any old barrel. Which means either you do it yourself or you pay someone to do it for you, which is very expensive, costing anywhere from $40-150 depending on the length of the barrel.
Another way of creating consistency is to create a consistent air seal. Meaning the same amount of air is coming out every single time. One way of doing this is putting silicone lubricant on the o-rings inside the gearbox. This makes them puff out more which makes a tighter fit allowing no air to escape. Another way is to replace the parts creating the air seal with better quality stuff and dual o-rings making it nearly impossible for air to escape.