Firstly, try and get you local site to chrony your marker for you, to 280 fps.
Then when you play, take a paintball, and the barrel to your marker (without the marker attached to it) and place the paintball in one end.
If it rolls out really quickly, then the barrel is too big for the paint.
If it gets stuck, blow from the threaded end,
if it comes out easily, then its a good fit.
If you have to blow hard to get it out, then the barrel is too small.
If your having problems getting the marker to reach 280-fps, make sure that if the piranha has a regulator on it (the fron grip)
that its letting enough gas into the marker.
If it isnt, then you will need to get an allen key, and put it through the bottom of the reg, and turn it anticlockwise half to one turn, then try and re-chrony.
give us a shout if you have any other problems