my “test” was one hour of gameplay but it did operate the way you noted–the first health bar (e.g. shield on traunt) went down about 10% quickly, then the gun really just acted like a purple Q system. I switched to a next two mags bonus radiation on ASE version and it worked a lot better.
If the 300/90 really is meant to only drop 10% and then do nothing (flesh or otherwise), it seems like a fairly useless anointment.