Here is the new double design magazine from We-Tech.  This updated version features an enlarged gas tank and is way more gas efficient than the old version.  It can hold up to 80 6mm BBs and will comfortably take green gas for normal operation which will last a long way before you will actually need any re-filling. 

Features:

• Heavy duty metal and polymer construction
• Dummy magazine functions as an auxiliary gas reservoir for improved performance
• Realistic looking 5.56 dummy rounds
• Large 80rd capacity
• Powered by green gas
• Compatible with WE M4 / HK416 / L85 / SCAR-L / R5C / T65 Series Airsoft GBB