g.willikers,
I can't name a cheap plastic that's not a polymer. Polystyrene is probably most sensitive to solvents among common plastics, and it is a polymer material.
The first paragraph of the
Wikipedia entry on polymers explains them well enough. They are just long molecules in which smaller groups called monomers are repeated by linking them together. It's an extremely common type of material configuration. A solidified epoxy is a polymer. Even DNA is a polymer. AFAIK, if you see the prefix "poly" in a plastic's chemical name, it's a polymer.
I'm thinking maybe you meant a polyolefin, or some other specific resin family that has good solvent resistance. Bottom line, though, is you are exactly right that solvents need to be tested on plastics in an obscured space to see if they will affect the surface. Gun makers select plastics to be solvent resistant, but that doesn't stop solvent makers from coming up with stronger solvents. I'd be careful with the spray solvent type gun cleaners, in particular.