If you are looking for a range upgrade try a new hop rubber and maybe a bucking. I like the maple leaf macron buckings but have a look around and see what you want, everyone has their own preferences and most of the aftermarket ones that are decent quality will work. Your hop unit and hop rubber are responsible for probably about 90% of your range and accuracy. Barrels can sometimes make a difference if the stock on is bent or broken or really low quality, but most of the time they are fine and not really worth the upgrade price/performance wise. If you just want to then I would suggest get a stainless barrel like the ZCI inner barrels. They are cheap, good quality and stainless is a lot more resistant to damage/wear than brass or aluminium barrels.
if you value reliability then I would suggest leaving your gearbox stock. Unless you are unlucky or have a serious amount of wear on the gearbox then stock will be most reliable. If something breaks then you could look at upgrading internals, but if you have never done it before you are more likely to make your replica less reliable rather than more reliable.
If you want to start learning to work on your airsoft replicas, I would suggest getting a second cheap gun to do it on. Leave your best gun stock as a backup and mess around with something you can afford to break while maintaining a stock reliable gun. You don’t want to miss out on a whole day of airsoft just because your only gun broke. In my experience it doesn’t matter how well your gun is built something will go wrong at some point and you will be grateful for a reliable backup gun. There is always the temptation to push everything to the limits if you are ‘upgrading anyway’ you might as well go for max performance…this inevitably leads to a less reliable gun (don’t ask me how I know ?)
