What I have been running is
Flak Jacket, Light Weight
Emp x 1
I just use the rifle as a very traditional mobile hard scoping class. I tend to use controlled aim shots rather than spraying. I just like the weapon better than the ballista or DSR because I can take on multiple targets quicker than with the bolt action rifles. The b23r is just a starting weapon because I don't like to get caught with the rifle in close combat. I usually will swap it out for the first shotgun or SMG I find on the ground. That is the primary purpose of fast hands in the class. C4 and the emp grenade are just general problem solvers. Again any advice would be welcome.
The Ballistic CPU is a big help in the XPR. It has some wild up and down swings while ADSed which are greatly lessened with the CPU attached. Past that other perks, etc really depend on ones playstyle. I have always used it as a semi-auto Ballista. It has roughly the same OHK box as the Ballista. So aim high and time your shots. As with most semi-auto guns going full auto with it is counterproductive. And as Sitrep has suggested a good pistol back up will save your butt.