A 200–300 GHz high power pulsed gyrotron oscillator has recently been operated in a 14 T Bitter magnet. The design of this pulsed gyrotron is based on continuous wave (CW) constraints. A single cylindrical waveguide cavity with linear tapers on each end was tested using two magnetron injection guns (MIG). The first produces a large electron beam which excites whispering gallery modes and the second produces a smaller beam that will couple to volume modes. The highest output power of 970 kW was generated at 229 GHz in the TE34,6 using the large MIG with a 59 A, 92 kV electron beam. This corresponds to an efficiency of 18% which was the highest produced in this mode. Similar efficiencies were obtained at 202 and 213 GHz using the same MIG and at 290 GHz using both the large and small MIG. The experimental power and efficiency is about a factor of two below the single mode theoretical predictions, even at low current. A detailed parameterization of the TE34,6 mode's operating range, measurements of the beam's velocity ratio (α), and comparison to previous high frequency work at MIT imply that mode competition is one important cause of the low experimental power and efficiency.