During last year's Canobie paintball park's Big Game, I had brought an entire case of marbs down with me. Was fresh in from RPS, kept in warm/dry conditions for the two weeks I had it, opened the bags just long enough to fill pods and a hopper... and I gauged it out on my empire kit to .687.
On my 'mag, it shot the marbs just fine, no problems. I switched over to my model 98 w/a flatline, and every single ball I fired exploded in the barrel. I was dumbfounded!
Walked off, (dry) cleaned the barrel, checked it, reassembled, and went back out. Same thing. Cleaned the barrel again, borrowed a pod of All-Star from a friend, and it fired fine. Checked that on my empire kit, lo and behold, .687.
I had only ever fired PMI premium through it before - I figured marbs would be a little easier to get breaks at long range. Not a single ball got more than 10ft from my barrel. I never could figure out why marbs did that, and to this day I havn't tried them again.
In the end, it didn't really matter - the referees at canobie had absolutely no concept of safety or game management so our twenty man group left dissapointed. Something about two refs trading a full face mask for a mouthless mask on field in the middle of a firefight so the other can smoke a cig... doesn't leave a good impression at all.