Vulcan 2 not shooting after taking off barrel and reinstalling it

I know I did something wrong but what. I had 2 pellets jammed in the barrel of my Vulcan 2. I went on Youtube and found out how to remove the barrel. Very easy, but after clearing the barrel of the 2 pellets, I re-installed the barrel. No problem so I thought, now to go shoot. Upon cocking the gun and pulling the trigger, all I got was a poof sort of sound, like very weak. I tried reloading and again just a poof sound. The pellets did go out the barrel but they only went a feet I think. I removed the magazine and dry fired it and the same thing happened. I am very frustrated to say the least. It must be something simple I hope. Any help would be very appreciated. Thanks.
 
No I didn't. I am new to PCP's, is this O-ring attached to the barrel? It looks like there is a recessed rim on the end of the barrel and I was thinking that maybe an o-ring would fit there? I am taking a picture of what I mean. It funny but I looked and looked for anything that may have fell when I removed the barrel. Nothing.

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