just for clarification, i'm pretty sure that range doesn't come from barrel length. the barrel only determines the BB's accuracy and path of trajectory. it is mainly the power of the spring that determines the range.
if you are adding a 650mm barrel, you might want to consider a "bore-up cylinder" kit to reduce the suction like back pressure.
if you can spend more money on the gun, and I would assume your purpose would be sniping (hence the 650mm barrel) then i suggest you adding a stronger spring than guarder 110, tougher gears, aftermarket spring guide metal bushings, and the bore-up cylinder kit
http://www.redwolfairsoft.com/redwol...il?prodID=8406