The spring guide is a component found in an Automatic Electric Gun (AEG) gearbox (also known as mechbox). AEGs function like spring powered guns internally where you have a piston forcing air through a cylinder and out of the gearbox to drive the BB out of the gun. The piston needs to be pulled back and cocked and released on the trigger pull. The difference between a simple spring powered gun and an AEG is that an AEG does what the spring powered gun does but without having to be manually cocked and it can do it in semi or full auto by using a gear and motor system to do all the work. Behind the piston is a powerful spring which is going to dictate how hard the gun is going to shoot. Players can upgrade or downgrade their AEGs by swapping springs to dictate the velocity that their guns will shoot. Holding the spring in place is the spring guide. This keeps the spring from popping out of place during compression and causing jams within the gearbox.
Standard spring guides will typically be plastic or cast metal from the factory. Spring guides are not known to break but it is not in the realm of impossibility. Spring guides can be upgraded to improved designs such as ones that have bearings. The bearings increase the life of your spring by preventing the spring from binding. The bearings do this by providing some give for the spring as the coils may want to naturally turn and rotate when under tension. One byproduct of running an aftermarket spring guide is that sometimes these bearings increase the tension of the spring and can increase velocity a tiny bit. The space occupied by the bearings will pre-compress the spring.
A spring guide is pretty simple so you can pick whatever one works. There is a potential for failure for multi-piece units that are built poorly but this is exceptionally rare. The same goes for the bearings but these don't spin at a tremendous speed, so again having a failure here is pretty rare and exceptional. Remember the more complexity you add to a basic part the more likely you can induce new problems when things wear out. We've been working on our guns for many years and just when you think you've seen it all there's always something new.
If you need gearbox work done please consult with someone experienced in the matter. Gearbox work is very tricky and we can guarantee you it will not go as planned on the first try. If you are looking into learning to tech though, there is a wealth of resources online with many video guides from the community on how to work on an airsoft gun. We recommend not messing with your good working gun and instead purchase a second gun and try to learn on that. We would recommend learning on the M4 platform since the M4 with version 2 gearbox is the most prominent design out there and you can problem solve and get parts for them pretty easily.