Pour your ammo into a bowl, pour in cola (yes, Coke or Pepsi) to cover the BBs, let them sit for a while, stir with your finger every 10mins or so. Then rinse, pour the BBs into a wool work sock, with some laundry detergent, close the sock with a zip tie and toss into a load of laundry. Pull the sock out, rub the BBs in between your hands (still in the sock mind you), run them through another cycle. Pull a couple out, dry off and try the rub test, repeat if necessary. This is how I've always taken the graphite off the Straight BBs.
And I usually wash all my sniper ammo in dish soap to clear off the slight oil finish on them, give more friction on the hop up rubber, which means you can have a lower hop up setting, leaving you with less of an obstruction in the barrel.