Due to their capability for supporting high-peak powers, large-core hollow optical fibers may be used to deliver high-peak-power nanosecond pulses for the fluid flow measurement technique of particle image velocimetry [Meas. Sci. Technol. 16, 1119 (2005)]. One drawback of using such fibers for fluid flow measurements is that the output suffers from a speckled interference pattern due to the fiber's multimode nature, which can lead to a loss of spatial information and reduced data quality. Presented here is a technique to reduce the speckle contrast from these fibers when delivering nanosecond pulses. Significant smoothing of the output intensity distribution is demonstrated, giving an improved source of illumination for fluid flow measurements and other imaging techniques requiring pulsed laser illumination.