On the packaging of the PDI .01 PTW 509mm inner barrel there is this interesting note:
Tighter Dia. as 01, you'll get more highly FPS. larger Dia. as 08, you'll get more accuracy. 04&05 are effective for all range shots.
Now, despite the bad English, this makes some sense as the wider the barrel, the less BB bounces off the wall (at least on theory) but this, naturally, would involve loss of air.
The build material is advertised as SUS304 (stainless steel).
As a whole, the installation is a straightforward (once having overcome the darn barrel key) replacement of the PTW inner barrel.
Below you can check the minor difference between the .01 PDI and the 6.04 stock Systema barrel - the original inner barrel seems to fit somewhat tighter with the hop adjuster.

- pdi inner barrel fit 2.jpg (198.84 KiB) Viewed 2789 times

- systema inner barrel fit 1.jpg (198.4 KiB) Viewed 2789 times
Once I complete my poor overall PTW maintenance attempt, I will get down to actually testing the barrel and could post some results.