In this work, five fundamental concepts were combined to a low development of high efficiency, low divergence, narrow bandwidth, flashlamp pumped oscillators capable of operation over a broad operating range. These concepts were: flashlamp pumped Nd:Cr:GSGG to achieve high efficiency, a 'Reentrant Cavity' to eliminate birefringence losses, a variable radius back mirror in a hemispherical cavity to achieve maximum Gaussian beam fill factor, a very high damage threshold, spectrum narrowing output coupler fabricated using a stack of uncoated etalons to form a resonant reflector, a cylindrical zoom lens to completely eliminate astigmatism. The results were successful, and yielded an oscillator that produced 10 mJ, TEM00 300 MHz bandwidth, 75 ns pulses, over a repetition rate of 1-20 Hz, and at a slope efficiency of 2 percent. These techniques were also successfully applied to a YLF oscillator. They may, in part, be adapted for use to unstable resonators.