Mammoth Uncensored Word List:
48 words ending with er, 15 letters long

astrophotometer autotransformer chromatographer chronobarometer cigaretteholder cinematographer contrapropeller copyrightholder counterattacker counterreformer crossclassifier crossquestioner diastereoisomer electrocutioner historiographer katathermometer kinematographer malpractitioner microcontroller microphotometer microprogrammer misunderstander mountainboarder nonmanufacturer nonphotographer overneutralizer paroemiographer phacoemulsifier photomultiplier photosensitiser photosensitizer phototypesetter phytogeographer premanufacturer radiogoniometer radiomicrometer radiosensitizer singlepropeller stretcherbearer subcommissioner supersensitiser telethermometer thalassographer thermobarometer transmissometer ultramarathoner ultramicrometer weatherstripper