Bigger is not always better. From everything I have read regarding the VSR-10 design it seems the stock 430mm barrels preform the best due to the cylinder volume of the gun. Also, a bore size that small (assuming that is accurate) will most likely lead to a lot of jamming unless you swab it religiously. I would recommend getting a PDI, laylax, or polarstar 430mm barrel that is at least 6.01mm, preferably 6.03mm. On the flipside of that, keep in mind range is almost entirely dictated by your hopup. A good, well tuned hopup is the key to long range.
Also, if you have the tools, you can just cut your M170 spring down a couple rungs and it should reduce muzzle velocity. If memory serves 1 rung is about 5-10 FPS, so in your case I would cut the spring about 4 rungs. My M170 put me at 700 FPS
*EDIT* Also, now that I look at that barrel more, I noticed that it is only for the Systema PTW/CTW guns. It wouldn't work in your bolt, or any other non PTW/CTW gun for that matter.