Welcome to the forums. You don't state where you are based - so I'll assume UK.
My notes:
- Don't replace the barrel
yet (I don't think the AEP barrel will work anyway). The TopTech stock barrel is decent enough - just give it a good clean for now.
- Start with replacing the hop rubber and nub and see how it performs:
https://www.ak2m4.co.uk/internal-parts/hop-up-buckings-nubs/maple-leaf-macaron-60-nub
- This gun uses the older-style G&G hop unit, which isn't bad, just a bit outdated - maybe later replace it later with a ZCI Plastic rotary unit (when back in stock):
https://www.ak2m4.co.uk/internal-parts/hop-up-chambers/zci-rotary-hopup-chamber-plastic-m4
- Chrongraph your gun before and after any upgrades. There's a good chance that as-is, your gun will struggle to hit 300fps (TopTechs of this era were 1j/328fps from the factory to UK shores but often were below that).
- Consider a longer inner barrel/mock suppressor if you need to get the FPS up - the very short barrel means that the gun is over-volumised (hence the noise) which means a good deal of air is wasted in the firing cycle. A longer barrel will make use of that extra air and provide additional FPS as a result.
None of the above involves opening the gearbox. If the gun shoots well enough after the above, then happy days.
Given that the gun hasn't been used in some time, here are some other things to consider next if you are having any performance problems:
- If you are confident on opening the gearbox - clean and relube, particularly the compression parts (Piston, Piston Head, Cylinder and Cylinder head etc.)
- Replace the piston and cylinder head O-rings
-
Important Inspect the gearbox casing for any cracks around the front/Blowback port - Toptechs are notorious for this (example below). You can do this to some extent without taking the gearbox out of the lower receiver:
View attachment 111353
- Inspect the gears for any wear/damage
- Inspect the piston rack for any damage to the teeth (G&G Pistons aren't great)
I'm sure others will chime in shortly, but my advice is to keep it simple (and cheap) to start with.