I didn't notice the brick entry on Sarah, considering I was testing on a open test area before using a map-interior area for my testing ^^; Aditionally, testing both in and out of minigame revealed no bullet pen through a wall of 1x4x1 bricks, quite literally at reading your post.
That would be a nice feature though, as would trading between a 'Sniper' mode and a 'rapid fire' mode would add to it's uniqeness.
As for MM, it's got a pretty high spread as is, much higher then the LMG, meaning it's pretty much worse then (high spread) shotguns on long distance. Still, interiors it'd be a beast. It's got enough faults to keep it 'sane', though if anything my suggestion would either be a speed decrease on the player whilest firing or a tiny reduction of refire. Then again this is just my opinions, others may have other ideas.