I'm not 100% familiar with all the various spring sniper rifle upgrades out there, but I do know that you'll be basically looking for Maruzen APS2 parts for your UTG L96. Keep in mind, that 99% of AEG upgrades out there will not work for your UTG L96.
Regarding the diameter of the barrel, I generally see 6.01mm being the standard for upgraded long range spring sniper rifles.
Here's a barrel that I believe should work that is 6.03mm:http://www.airsoftgi.com/product_info.p ... ts_id=4536
However, generally you don't want to skimp on the inner barrel for a spring sniper rifle, since accuracy is basically the whole point of using a sniper rifle.