You're referring to United States Ammo Co. of Tacoma, Washington?
The USAC plastic cartridge is not reloaded the same as metallic cartridges.
The USAC design uses a heeled bullet made to "snap" into the USAC case using a reloading tool like a pliers. You popped out the spent primer, used the enclosed USAC dipper to deliver a specific charge of a specific powder (Unique, I think) and then you popped the copper-plated USAC bullet into the case. You could not use anything but the charge USAC specified because the plastic cases were not meant to withstand anything more.
The components (other than powder & primers) were not made to be used for anything but USAC loading. You can not use any other bullet than the plated USAC bullets with the USAC cases due to the heeled bullet design which was necessary because of the case wall thickness of the USAC plastic cases.
USAC has disappeared from the market long ago and sometimes the components still turn up from attics and basements.
A local gun store near me discovered a pallet of USAC bullets in various styles and was blowing them out for a very cheap price. It was said that they can be loaded in regular metallic cases by turning them backwards and having the heel portion forward like an odd SWC design. Or you can load them right way out but they have to fit very deep in the case almost like a wadcutter and needed reduced charges. Good enough for a plinking round.
On the Ohioans for Concealed Carry forums we had a discussion about 38 special bullets and the USAC bullets came up, one of our members loads and shoots them in metallic cartridges. He posted a few pictures of his USAC loads if anyone wants to see them.
http://ohioccwforums.org/viewtopic.php?f=22&t=48059&start=0&hilit=usac