An Active Well Coincidence Counter (AWCC) has been modified to measure variable enrichment uranium hexafluoride (UF6) in storage bottles. An active assay technique was used to measure the 235U content because of the small quantity (nominal loading of 2 kg UF6) and nonuniform distribution of UF6 in the storage bottles. A new insert was designed for the AWCC composed of graphite containing four americium-lithium sources. Monte Carlo calculations were used to design the insert and to calibrate the detector. Benchmark measurements and calculations were performed using uranium oxide samples. The Monte Carlo generated calibration curves benchmarked to uranium oxide resulted in assay values that agreed within 2 to 3% of destructive assay values. In addition to UF6, the detector was also calibrated for HEU ingots, billets, and alloy scrap using the standard Mode 1 end-plug configuration.