However, it's possible to do this in other ways too. With the advent of the new Red Tiger machine gun (can't remember the full name), you can have a huge gun with a monstrous 100 round cartridge, 90 rpm, and terrible mobility. If you wanted to camp the bridge, this could be your gun.
Another way is to use a shotgun. However, this is only viable if using a clip-reloading shotgun (Saiga, Spas-15 etc) and Mag Upg A/B as the low magazine size will prove a challenge otherwise. However, it becomes exponentially easier to finish the game, even solo, with a shotgun with more than 20 shells a mag.
One time I finished a game with 4 other snipers, all using semis. Going for all headshots actually cleared the room pretty fast, as long as we stayed on the bridge.
Last tips: just aim for the heads, take out the exploding balls first, use second firing method (Plasma Blast?) when you get to Betelgeuse and you'll do reasonably well. If you need any more help, there are lots of Youtube videos to reference. Good luck!
sources: past experience and 4 Beretta Barbados perm