It's pretty simple. Marui Spec VSR cylinder is 23mm wide. WELL is 21~20mm wide (Depends on QC and batch). Well's is smaller,and so is the receiver and cylinder guide rings to support the smaller cylinder.
Parts compatibility is more of an issue. It's still got enough volume to function perfectly but Joule creeo won't be as profound as like on,say,a TM Gspec(Which is just absurdly overvolumed)
