Just wanted to also mention that while some stock guns may not need any changes for sake of performance, there's also the issue of reliability. I've seen several chinese guns that will outperform a high end one in both range and accuracy, but won't come close to the life span of one from Taiwan or Japan. Material quality is crucial when it comes to the gears and piston. Just because the box says "metal gears" doesn't mean they're case hardened steel like you'd find in a marui, CA,KWA, G&G or other high end brand. Some are a zinc alloy that is softer and wears more quickly. Some pistons and tappet plates are a lower quality plastic that is more brittle and therefore more prone to failure. I know this isn't news to you veteran player/techs, but new guys can get discouraged when their gun takes a crap in the middle of a firefight. The good news is when you do have to replace a part, there are after market brands that will last much, much longer.
