You could also try a Big Out H-Hop hopup nub. At only $15, it's a cheap experiment:
http://www.airsoftparts.ca/store2/in...roducts_id=720
Also, you don't want an M4 bore up kit with that length of barrel - go with an M16A2 boreup set - same nozzle/cylinder head/piston head as the M4, but it will have an unported cylinder (which you need with that 650mm barrel).
Search any retailer (redwolf, WGC, WarIV, UNCo) for part #ZA-03-34 (New Bore Up Cylinder Set - Systema).
Also, Juggles at airsoftparts.ca carries the Modify bore-up kits for M16A2:
http://www.airsoftparts.ca/store2/in...roducts_id=682
ROF too high? It's a sniper rifle, it's not supposed to be FA! Get a Triggermaster MOSFET (
http://www.airsoftcanada.com/showthr...trigger+master) programmed to semi-auto-only with a 3 second delay between shots. It will make your response time on the trigger pull almost non-existant. Then, make sure you get a battery with high discharge rate (I like LiPoly 11.1v myself) and you'll have virtually no lag from the time you touch the trigger until the shot is fired.