He sells the 10Kg Spring and O-ring combo used in the mod guide in his sales thread on the Nerf SG forum for $10SG.
In the mod guide SGN uses the same battery setup many are now using in their Stampede's to give 16.8v. I was planning on doing the same thing but I ended up just trying the C batteries in there first and to my surprise they worked.
I was very happy about this because I actually prefer the slower ROF as it means less reloading.
And now for ranges. I used unweighted, large snap cap, blue foam darts for my test and got an average of 23m flat (76ft). A couple (out of the 10 shots) went a couple of meters more again.
Bear in mind though, the test was done on the road so the darts will have bounced a little further than their original landing point on the ground.
In conclusion, SG Nerf is great to deal with, the kit is quality and well priced and it gets great results. A must have if you own a Swarmfire.
Sorry I didn't include any pics but I figured there were enough in the mod guide link up top.