Sniper custom ideas

If by "sniper" like you are referring to appearance, then stick a long barrel and a scope on it, wrap a bush rag around it and have at it.

However, you will find that performance wise the long barrel will do little for range.  Paintballs are limited at speeds of 250 fps to 300 fps (depending upon the field) for safety reasons so changing barrels will do little for maximum range.  Effective range is a different manner.  Almost any after-market barrel will provide better consistency than almost any stock barrel.  The J&J Ceramic and Lapco Bigshot are the most commonly recommended barrels on this forum and both are very good for the price.  As there are a lot of other options to consider I suggest you read through the stickied topics at the top of each forum page to get a better idea of what you want/need.

The J&J and Lapco barrels are just regular straight barrels that happened to be built to higher tolerances than the stock barrels.  While the Flatline will add range it does not provide single-shot elimination accuracy at those ranges because of the poor ballistic performance of paintballs and the decreased velocity that a paintball fired from any barrel would have at that range.  (There is less chance of it breaking.)  However, they make a good suppressive fire tool because with multiple shots the chance of a single shot breaking on target (at ranges other barrels are incapable of) is increased.

APEX barrel would be the way to go, you can "curve" the paintball, so you can curve it up if you want.