The HD stands for hard I believe, which is just a flat face.
The VC stands for vacuum, which is silly.
With that one there's a big hollow cavity in the front of the piston that is supposed to shoot air into your O-ring and expand it faster, but I don't think that it's worth the dead space in the system. I haven't heard actual use from it, but I think it didn't do anything special.
If you look at Maple Leaf, Laylax, or Action Army, or AEG pistons they have lots of little holes in front of the O-ring for the same purpose, but I think they work better than a huge hole since there's less dead space to ruin compression.
Not sure where to get piston ends, but PDI is rather hard to get ahold of these days so you may be out of luck. The Laylax pistons are good but are a bit difficult to get, but I think that the SSG10 weighted piston may be a go.
The VC stands for vacuum, which is silly.
With that one there's a big hollow cavity in the front of the piston that is supposed to shoot air into your O-ring and expand it faster, but I don't think that it's worth the dead space in the system. I haven't heard actual use from it, but I think it didn't do anything special.
If you look at Maple Leaf, Laylax, or Action Army, or AEG pistons they have lots of little holes in front of the O-ring for the same purpose, but I think they work better than a huge hole since there's less dead space to ruin compression.
Not sure where to get piston ends, but PDI is rather hard to get ahold of these days so you may be out of luck. The Laylax pistons are good but are a bit difficult to get, but I think that the SSG10 weighted piston may be a go.
