Legally, it can't be done.
Oh? Has anybody ever been sued for using them in their games? As far as I'm aware, it's not done anymore purely out of fear.
And even that's a lie: a bunch of games still do it. Just not ones by Obsidian, evidently.
It probably wouldn't be hard to get permission either, if you were a real paranoid, since it's decent marketing for your guns, like YouTube.
Then again, music companies keep trying to sue YouTube, so I think I'm giving these guys too much credit.