Sniper Barrel comes loose on 98 custom

I've got a tippmann 98 custom with a snipper barrel attachment (i think its called flatline). like the one in the image. Everytime i shot a paintball the barrel falls off like if it was loose. I tightend it alot and the same problems occures. Ive got two other barrels, a speedball barrel and the original. To switch barrels from the snipper attachemnt to the regular do i need to get a diffrent attachent to the gun ??

This is what i have now:



And i want it to be like this :


How do i remove the sniper attachment and replace it with the original barrel ??? and no i cant screw it on, i think i need an attachement ??

K Hop wrote: unscrew the screw that is on the flatline adapter at the front of the gun, where the barrel goes into the gun..