How would this stuff work with plastic fouling in a shotgun?
Unknown, but I doubt it would work very well on it.
Certainly not as well as a modern shotgun solvent that's been formulated specifically to remove the plastic fouling.
The old US bore solvent was specifically formulated to clean US Service Rifles fired with corrosive ammo.
How well it'll work on copper fouling and/or plastic fouling is unknown because it was never intended to be use for that, plastic shells not existing at that time.
If you insist, buy it and try it. Worst case is you fail to get your guns properly cleaned, and waste a few dollars.
There's a reason the old solvent was abandoned by the US.
First, it's a health hazard.
Second, better solvents were developed for use with modern ammo.
Last, it's very possible the old solvent has a shelf life. It may well have deteriorated and no longer work properly, or who knows, it may actually be harmful now.