Let me preface this by saying that this Vet and I got off on the wrong foot when I first received it. I came close to parting with it, but I stuck with it and started uncovering it's potential, while realizing just how well made these things really are!
I spent a couple of months in my spare time this winter playing around with it, and eventually figured out what I needed to change. I got busy prototyping custom parts for it, each one undergoing several revisions until I was satisfied with the level of perfection with each one of them.
Here's my current configuration:
Current Barrel: 600mm FX STX Barrel (slug A) + Custom FX barrel kit adapter
I can get away with the length of the 700mm without too many issues, and I can also generate tremendous power for using the 500m, but 600mm seems to work best for me as a hunter. This particular 600mm kit is from the FX Crown that I bought from @Bob_O a while back. That barrel had always been real good for me, and I knew that it would be solid for eliminating anomalies during my testing.
Here is the 600mm without the Taipan moderator..
and another random photo with the 500mm barrel kit + DonnyFL moderator (Tatsu or Sumo, not sure)...
Adapter...
This took some time, trial and error to materialize. I started off with a plan for making a drop-in replacement for the FX brass TP inlets, but that left me with a really flimsy adapter. I grabbed some graph paper and sketched out some ideas, then made them on my lathe. The next version that I made was a full-size replacement for both the stainless adapter and brass inlet (combined into one piece). This version (aka v1.0) featured an integrated seat for the barrel liner. I did comprehensive testing with the v1.0 (and a v1.1), but determined that the projectile seating depth was not right with the gap between the Taipan's little aluminium transfer port piece and the liner. In v2.0, I eliminated the seat and made some other small modifications to get it perfect. Here are v1.0 and v2.0:
More of v2.0...
This adapter has made me VERY happy. Harmonics are not an issue for me, I suspect the solid brass adapter is doing a good job of absorbing a lot of the shock.
Taipan Moderator
I was dead set on using the factory moderator. I swapped the muzzle thread out with Donny's 1/2-20 adapter, and when you combine the Taipan moderator with anything = it's the quietest thing that I have ever had! That said, an interesting challenge that I ran into was figuring out how to use the Taipan moderator with the FX STX barrels. The FX barrels are ~13.5mm OD and The Taipans are ~15mm. Luckily, the two can mate since they're both 1/2-20, but the inside diameter of the moderator is larger than the OD of the FX sleeve - leaving a big gap between the end of the moderator and the FX barrel sleeve. Naturally, this would cause the shooter to get shot in the face by air and lead dust (and no, I wasn't dumb enough to fall victim lol)
The simple fix for this - I kept the shaft collar from the inside of the FX barrel shroud, stuck a ~14x1.5mm o-ring in front of the collar, so it gets sandwiched between the back of the moderator and the collar (thus sealing the gap). It doesn't look pretty, but it's perfectly functional. Eventually I will prettify it, but it's not a priority for me at the moment.
Custom "Fatboy" Valve System
I'm incredibly impressed with how this new valve system has turned out. The power gains are substantial over the factory vale system (90+ fps!), and the shot count increased on average by 11 shots per fill.
For this valve body, I'm also using the harder 485 marine brass (more scratch-resistant than the softer factory valve bodies), and the valve stem is made from (Ketron) PEEK on a tapered 6AL-4V titanium valve pin (weighing 1.1 grams!). The sealing surfaces are using an interference fit with offset angles, which provides MUCH better sealing properties between these two relatively hard materials (you typically see a hard and soft material with valves, but these two are like diamonds lol). The head on this valve body provides much better valve compression and performance (consistency, efficiency, and shot count). The length of this pin has also been optimized for the stroke.
For comparison, the factory valve is on the left, the middle one was another prototype, and the Fatboy on the right:
Two 20cc Power Plenum Extensions
Rocking two of Ernest Rowe's Taipan power plenums for this build. I can get away with not using any, but I've chosen to use them when I have a 600mm or longer barrel installed
Hammer System / Springs
This is where it gets interesting.. I had defaulted to a big heavy spring like the other power tuners. Then, I decided to spend some time on testing different springs, brass weights, and polymer spring tensioners. I found that I can achieve 1-2FPS ES using a smaller spring, with a 3 gram hammer weight (with the little black POM-C guide at the rear) at the cost of limited FPS tuning range. The factory spring with the brown POM-H / Delrin spacer / bushing not only produces as much power as the heaviest spring that I could fit, but it also allows me to tune this Taipan down to as low as 315FPS! LOL .. quite the power range from simple HST adjustment!
Regulator
I'm running a Huma reg in my Vet. It provide a little more room for air, and I can tune it easier than the factory reg. There's nothing wrong with the factory regs, this is just personal preference.
More to come..
). I should probably thank Derrick as well for enduring my text messages during this build, while I worked through some if the ideas 
.. 37F, very wet the wind at 50-60+ MPH all day long (we're having a nasty Pacific storm here in Washington state today). I didn't want to drive out there for nothing, so I nailed a target at 100m and shot some groups anyway 