Fort Bribie was defended with two 6" breech loaded Mark XI guns. Their gun histories tell an interesting story. When rearmament of the fixed coast defences was mooted, 6" dual role guns were chosen. Both the Royal Navy and the Royal Australian Navy had stocks of superseded 6" mark XI ("Mark Eleven") guns on P Mark VI mountings surplus to their requirements. Six-inch batteries were eventually installed at Signal Hill (Sydney), Breakwater and Illowra (Port Kembla), Cowan and Bribie (Moreton Bay), Emery Point and East Point (Darwin), Rottnest Island (Fremantle), Paga Paga (Port Moresby), Goods Island (Torres Strait) and Kupang (Timor).

Six inch breech loaded Mark XI gun

These guns and mountings were designed for naval service and had all originally been installed in destroyers and light cruisers as the main armament of larger ships. They were designed in 1905 and were first brought into service in 1906 after construction at Coventry Ordnance Works Ltd. in England. Their characteristics are listed below:

Bore (calibre)	6" (152 mm) diameter
Weight	8726 kg
Length Overall	7.867 m
Bore Length	7.620 m (300 inches). Hence 50 calibre (50 x 6")
Chamber Volume	33.27 litres (2030 cu.in.)
Rate of Fire	5-7 typically (max. 12 in testing)
Projectile weight	High Explosive type (HE) - 45.3 kg (100lb)
Propellant Charge	HE (TNT) - 14.6 to 15.0 kg
Bursting Charge	HE - 6 kg
Projectile Length	HE - 58.2 cm
Muzzle Velocity	895 m/s (2937 fps)
Range	13085 m at 15° elevation (ship mounted); 16000 m maximum (coastal defence)
P VI mount	Training: +150°/-150°
	Elevation: -7°/+20°
Total weight	18.3 tonnes (mounting, shield and "piece")