A bit more than a year ago, I found myself in position of a good condition (i.e. barely used) FN303, complete.
I immediately had the valve off to my local machine shop for cutting the laser welds and, turning me a custom, delrin insert that fills the entire volume except for the minimal amount needed for proper velocity and, ensuring a smooth rear to front air flow to the powertube.
I then immediately set about getting more magazines (I wanted to build up what I consider to be my required inventory as I know these things can be very hard to get). Once I had the magazines I needed (late march of this year), I then set about setting it up for paintball.
Tymcneer (Brass Monkey Customs) found that the machine shop took just a hair too much material from the valve and it was causing me problems. He fixed that problem and, installed a ULT trigger to give me a short, crisp pull. I also had him re-thread me a Hammerhead barrel to get rid of the fin, and adapt it for Autococker threads. I also had a custom raincover fabricated. I set it up for a remote line and, duracoated it in my custom camo pattern:
I set it up initially with an HHA Optimizer, a Leupold Laser Rangefinder, and my custom 22x Zoom GoPo camera (I intend to capture video of those targets I shoot at ranges farther than 40yds), held in place by Inception Designs' Picatinny GoPro mount and, a custom offset made by Olane over on MCB:
A few notes about myself that are somewhat relevant:
For three years now, I've shot nearly nothing but FS rounds. I've published lots of ballistics information about .68, .50 and FS rounds and how to apply external ballistics to our hobby. I also maintain an FS Field listing (over on MCB) for others like myself.
Psst- I entered Totmacher 13 in the MOTM, if you like it, throw it a vote