If you're shooting 300... I think that's a bit low. Don't those come stock ~360 FPS on .20's?
Cheapest upgrades would probably be hop-up unit, bearing spring guide, and stronger spring, piston, and cylinder head. Although... to be honest things do add up pretty quickly.
If you want accuracy then the hop-up unit and (soft) rubber are probably what you want first, then a tightbore (you will see an increase in FPS and accuracy) I believe 6.03 or 6.04 tightbores are what's recommended (no tighter than that).
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