I actually like Modify and Guarder parts a lot. Great price/performance.
With Guarder they have a cylinder enhancement set which comes with everything for your compression unit. Piston, piston head, cylinder, cylinder head, and air nozzle.
Otherwise I'd go with SHS Gears/Piston and Prommy everything else.
The Madbull V2 is the barrel without that coating that scratches off right? It's good but I like the Prommy and PDI barrels a lot more but keep in mind you're gonna have to pay for that kind of quality.
Here and
Here Note that they're both out of stock ATM so you might have to go shorter or longer (and cover it up with a longer suppressor).
Otherwise Madbull and Guarder are good as far as price/performance goes.