An M4, M16, 416 style gun will use a Version 2 gearbox which is the most common. Upgrade parts will not be an issue to find; however if you are seriously considering spending 350 or more on a rifle I highly suggest you look at VFC. It is the preferred high-end platform and has fewer compatibility issues with upgrade parts. If you dig around online you can find a VFC VR16 starting at $350 canadian depending on the model. A cm16 is a good beginner gun when matching price and performance, although I can't say I've been very impressed by the Combat Machines I've tried.
Word of caution Hero's tends to have inflated prices, so shop around before you commit.
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