FAST Hop up for Silverback SRS. FAST hopup for Silverback SRS-A1 (Field Adjustment SeTting Hopup). 6061 aluminium CNC machining hopup unit for the Silverback SRS-A1. This new design allows you to adjust your hopup in-game without any tool, in just few seconds. This hopup utilizes standard AEG hopup rubbers and AEG barrels.