I'd add, besides starting by asking for help; start by playing first, for a year or so.
When you actually NEED an upgrade, trust me, you'll know what it is. Even the plastic bushings will be fine in a stock gun for that first year.
Different people like different things, and different guns have different needs. A metal body fixes a lot of things on a G3. Maintaining your Colts to prevent barrel wobble is unique to Colts. A lot of people like/use all sorts of optics. SMGs/CQB guns don't get upgraded the same way as long range rifles. Etc.
Again, you'll know what the gun's shortcomings (or strengths which you want to enhance) after you've played a few times. Quite a few times; there's no upgrade to fix crappy players, so make sure the problem actually does stem from the gun (accuracy comes to mind for this one).
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