In some examples, a non-incendiary tracer projectile structure for housing an electrically powered illumination source comprises a jacket (100) comprising a generally pointed forward end (103) and a generally blunt rearward end (105) defining a rear opening (107), an internal retaining structure (109) disposed towards the rearward end of the jacket to support a protective window (111) that is transparent to a selected electromagnetic wavelength, wherein a portion (113) of the rearward end of the jacket tapers in a radially inward direction whereby to define a circumferential narrowing of the rear opening so configured as to mechanically protect the internal retaining structure.