Air seal packing and how to fit

I think this bit has fallen out of my barrel, any idea how to refit it

its this part I think

http://www.celciustechnology.com/body.php?id=items&iid=BRL-029

Cheers

Easy, but you have to strip your barrel.

This rubber O ring - simlpy goes onto your barrel and in front of the brass colar that pushes up inside the hop chamber so it closest to your hop - dont push it too hard as you dont want it to push your hop unit off the end of your barrel.