This has been shot at so many times
Already, make it system choose a system.
KJW -OOTB most reliable, Thermold magazine, NO CO2
WE Open Chamber -Essentially like except STANAG magazine, Nozzle are prone to break when double/triple feed
WE AWSS -Fewest moving part (less part breakage), long term requires trigger set upgrade. Hopup is kinda finicky
Magna System -Has the most aftermarket parts. Long term the most expensive