A round robin of testing was performed to compare the performance of rotational dynamic mechanical spectrometers being used within the nuclear weapons complex. Principals from Sandia National Laboratories/New Mexico; Lockheed Martin Y12 Plant at Oak Ridge, Tennessee; Los Alamos National Laboratory, New Mexico (polycarbonate only); and Honeywell Federal Manufacturing and Technologies (FM and T), Kansas City, MO, performed identical testing of hydrogen blown polysiloxane S5370 and bisphenol-A polycarbonate. Over an oscillation frequency sweep from 0.01 Hz to 15.9 Hz at 135 C, each site produced shear storage modulus values with standard deviations of less than 5%. The data from Sandia, Y12, and Kansas City agreed to within 4%, while the Los Alamos data differed by as much as 13%. Storage modulus values for a frequency sweep of the S5370 at 35 C had standard deviations between 6% and 8%, and site-to-site agreement averaged 3%. The shear loss modulus values had standard deviations of 5%, 7%, and 52% for the sites participating, while the results differed by 12% on average.