Since you've said in another post that this was a new purchase, from a retailer (patrolbase), and it shot 300 bbs before it fried
- then I would send it back for repair/replacement (as stated by CrackCommandoUnit1972
Then: just use the gun until it needs upgrading
Cyma plats tend to already have upgraded gearbox internals and motor, so you're good there.
Otherwise, pretty much what Leo said
Until the mosfet dies, wouldn't bother replacing
Until the hop rubber shows signs of wear, (usually after a few thousand rounds), wouldn't bother replacing
You could upgrade the barrel, but, if it's firing fine - leave it
Mosfet wise, with the perun:
You might have to modify the gearbox shell slightly for it to fit in properly
mostly just means filing down/removing a "post" or sometimes the cut-off screw hole.
If this is the case, then you'll need to clean out your gearbox shell afterwards and reapply any necessary greases
- cracking open the gearbox voids warranty
Start by contacting Patrolbase, tell them the problem with the gun, they'll either try diagnosing/fixing the issue remotely, or request you sent it them for repair
Then, if the gun works, leave it be
(so long as you havent opened anything, you can send the items you bought back for full refund, though maybe keep the silicon oil for general maintenance).