Y U NO LISTEN 2 US!
Haha, just kidding (or not).
There's one thing you should know about barrels, it's the inner finish that makes or breaks a barrel, not the bore diametre..
That's mostly for fine tuning the FPS/Joule output. Now, if your gun is right on the limit in its current form, by putting a tighter bore in you increase the energy output over the site/legal limit.
The term "precision" is faff, all things considered.
As far as hop rubbers go, there are dozens to choose from. I personally favour Maple Leaf rubbers (AEG MR-Hop), if they don't fit then I resort to Prometheus Purple ones. The Prometheus Blue rubber + flat tensioner combo is also great, albeit a tad expensive imo (I remember paying €15 for the rubber and 16 for a pack of 2 tensioners) compared to Maple Leaf (€11 for the rubber, 3 for the Omega tensioner)..
Some people swear by G&G greens, but I only had issues with them so can't recommend those.