Stupid Airsoft reviews the Tap Airsoft HPA adapter for the KWA F90 airsoft rifle. The adapter allows the F90 to use cheaper, higher-capacity AEG M4 magazines and HPA, which performs better in cold weather. The adapter is a sturdy, 3D-printed part with a built-in magazine ejection system and HPA kit. It's compatible with standard AEG M4 magazines and allows the F90 to fire in semi-auto and full-auto. Pros include cheaper magazines, higher round capacity, and better cold weather performance. The cons he mentions include the need for an HPA tank and line, a non-ambidextrous magazine release, and a slight loss of BBs upon initial magazine insertion.