SRS Hop Up Unit & Flat Hop Up Rubber - CNC machined 6061 aluminum hop-up unit for Silverback SRS-A1 Sniper Rifles. Accepts AEG hop-up rubbers. Includes: 60 degree silicone rubber, 5x M3x4 screws, 19x2 o-ring, 8x2 o-ring, pre-installed 5.8x0.75 o-rings.