12″ diameter (304.8mm), 2 Bates grains, 260lbs (117kg) of propellant!
Be interesting to know the thrust and chamber pressures achieved in this test, they seem to have figured out any grain cracking issues that could arise and over pressurize the motor, looking forward to seeing more tests of this size in the future!
Video Caption: 2 grains 12×20
260 pounds of propellant
Motor by Rick Maschek and Eric Beckner