gas thru stock question

[AutoFiend]

what kind of gas through stock is this?

http://i181.photobucket.com/albums/x299/mpsd18/7d99d0fb.jpg

[tigar19]

I see a Special Ops logo on it.

[Hook]

That's the second generation Special Ops T2W stock. Air passes in the bottom tube, while the first generation stock had macroline passing inside the tube.