A holster shell is constructed with a horizontally oriented closed bottom wall, which along with opposing outward and inward facing sidewalls form a U-shaped cross section. A holster cavity is exposed along front and rear sides of the holster shell, through respective front and rear gaps that are defined between the sidewalls and the bottom wall. In some embodiments, the front gap of the holster cavity provides clearance for varying dimension, electro-optical or other sighting systems. In some embodiments, the rear gap of the holster cavity provides clearance for varying dimension laser sights and/or flashlight modules. Handgun retention features, such as trigger guard and/or ejection port conforming profiles, are defined within the holster shell. When a corresponding handgun is inserted within the holster cavity, the holster shell provides retention bias against the handgun.