This study investigates the influence of key parameters on in-service bullet impact resistance of ultra-high performance fibre reinforced concrete (UHPFRC), with the aim to provide design guidance for the engineering applications. The effects of steel fibre type and dosage, matrix strength, coarse basalt aggregates, and target thickness are researched by subjecting the UHPFRC to a 7.62 mm bullet shooting with velocities of 843–926 m/s. The results show that the UHPFRC, designed by using a particle packing model with compressive strength around 150 MPa, is appropriate to develop protective elements considering both anti-penetration performance and cost-efficiency. The 13 mm short straight steel fibres show better anti-penetration than the 30 mm hook-ended ones, and the optimum volume dosage is approximately 2% by considering both the penetration and crack inhibition. Introducing coarse basalt aggregates with the particle size up to 25 mm into UHPFRC reduces the powder consumption from 900 kg/m3 to 700 kg/m3, and results in slightly higher mechanical strength and significantly enhanced bullet impact resistance with 14.5% reduction of penetration depth. The safe thicknesses (perforation limit) of the designed UHPFRC slabs are approximately 85 mm and 95 mm to withstand the 7.62 × 51 mm NATO armor-piercing bullet impact under velocity 843 mm/s and 926 mm/s, respectively.