Thanks Uriah!
Do you get more XP with a full session? I actually avoid the full sessions because inevitably somebody will jump in and start stealing my kills. I usually jump around servers until I can find one with just 4 or so people in it.
Yeah, I'm pretty sure the amount of people determines your BONUS SCORE - which boosts your overall XP up pretty dramatically.
On a private session or a public session with < 5 people - most I ever got was 1432 XP.
I'll bounce around until I find at least 10 people, then make sure no one is doing Twin Rocks. Most of the time, people will come grief you, or hit continue when you're trying to grind it out. But, I found that maximizing your XP per attempt is far better than completing it more times. Keeps you from getting bored.
Another thing you can do is find someone who is level 50, and ask if you can swipe their Buffalo Rifle. Then most NPC kills are one-shot kills. That helps too.