An ACM Brand, TFC or Tyen Fong released an M4A1 AEG with Militaru Spec Real Steel Enhancement parts. Available at eHobby Asia, this costs US$175, which is a little bit higher priced than the other equivalent ACM AEGs and this doesn't come with trades. If you're ok with it, this package seems to be a robust one given the specs below and fires 360fps out of the box.

"TFC M4A1 Carbine Full Metal AEG

• Military Spec Real Steel Enhancement Parts
• Aluminum Receiver
• Full Steel One-Piece Outer Barrel
• Can withstand up to 70Kg
• 1/1 Scale High Performance Assembled Electric Powered Airsoft AEG
• Featuring both SEMI & FULLY Automatic Shooting Mode
• Full metal hi-cap magazine accommodate 300 Rounds of 6mm Plastic Bullet
• 6.03mm inner barrel (363mm)

• Separate Gearbox Design
• Shoot out 360FPS muzzle power nicely out from the package
• 5 Position Extendable Stock
• Real Size Handguard allows installation of 9.6V Mini Type Battery
• Open Bolt Cover to adjust HOP UP
• Metal Hop-Up Chamber
• Integrated Carry Handle Sight mounted on picatinny top rail
• Utilizes 6mm Plastic Bullet, this rifle give you a supreme accuracy on the target
• Sling mount can be installed with any single, 2- or 3- point tactical sling"