Let me just say that I have the same type of piston for my VSR, and I don't like it for one reason. After some use (1000+ or so rounds possibly), the o-ring will fall off and bits of it will come out your barrel. This happened to me because the big O-ring on my PDI tapered cylinder fell off as well. This caused the Piston's O-ring to slam into the cylinder head and be shredded into bits.
Don't recommend it for this reason. However, I still use the piston, but the metal piston slams into my cylinder head, which is really loud.