Do you remember how they made/sourced the 3/8" darts? Or what they used as barrels?
Same way we used to make any other darts: foam backer rod. Just in 3/8" diameter instead of 1/2". This was back in the glue dome days. No idea what people used for barrels. Probably brass, but PEX may have been considered also. IIRC, 1/2" PEX has ~3/8" ID.