If you have three friends that are willing to help out, search Endurance Arena CTF and you search the same but alone on your own team. Take Ammo Regen, do not take doubler! Take the Ammo Regen, go over to their base, and just keep killing them, your ammo will regenerate so you have infinite shots with the railgun. Just keep hitting them until you receive 250 of them. This can be done in 1-2 games is done properly. If you don't have any friends, Deathmatch Average/Endurance and you can get a crapload of them off of people, just keep at it.