The cylinder itself isn't APS-2 but all the insides of it are. It's actually very close to an MK96; weird cylinder but APS-2 compatible parts and MK96 hop up. The only companies I know of that make cylinders for the ASR are AirsoftPro who then sell to ASPUK and PDI.
My ASR is completely stock but I'm saving up for a new hop up and cylinder because I ripped my bucking and want a better hop anyway because ASR hops SUCK.