They balance everything out by merging some of the weapons/gear when they're used in PvP. For instance let's say I find a beast rifle that does alot damage in the campaign or strike mission and I do well with it so I decide to take it for a spin in the crucible. The damage will be reduced so that other plays with lesser weapons will have at least a shot. I'm not saying it's gonna be as weak as a little pea shooter, but it won't be as strong as it is in single player. I can't remember where I read this but I'm almost positive it was from a direct interview with a bungie employee