Well sir, you've answered your own question. The difference between regular game modes and Elimination/Skirmish is that there's a lot more team collision in the regular game modes. Because let's face it, Trials is the snipiest camp fest there is, and so hardly anyone is running blindly into one another. Meanwhile regular game modes has everyone running around wildly (In these CQC maps) and are more likely to make use of a shotgun. It's so easy to be going to point A and run into an enemy team member..so unless you're a no-scope pro, you're better off using a shotgun in these situations. So it's less the fact that shotguns are somehow superior to snipers, and more so the fact that the average player using a shotgun is more likely to get a kill because he's more likely to run into someone in CQ compared to "sniper range".