Tippmann A-5/Pro-lite/ProCarbine barrel threads. If you need/want an upgraded barrel, I have a used lapco bigshot 12" that I'm selling.
Yours has what is referred to (I probably mistaken) as a Euro grip frame.
I have an F4... it is not my favorite marker but it is kind of a classic. Has a very loud, sharp sound when fired. It its favor, it is very light and relatively easy to clean. If you need to get spare parts, I believe Bulldog Paintball was selling parts kits earlier this year and may still have them in stock.