Mammoth Uncensored Word List:
17 words ending with ms, 18 letters long

anticommercialisms congregationalisms constitutionalisms denominationalisms electrocardiograms electroretinograms latitudinarianisms postimpressionisms postmillennialisms predestinarianisms premillenarianisms radiochromatograms reconstructionisms transcendentalisms ultraconservatisms uniformitarianisms valetudinarianisms