The 3”/50 gun- Part 2

Early versions of the 3”/50 gun were discussed in the previous post.

As noted, something better was needed to counter the high performance aircraft of the day. The two great drawbacks of the earlier mounts were its slow rate of fire, and primitive interface with fire control systems. And as we talked about earlier, the Bofors 40mm in nearly universal use lacked the stopping power needed. The Navy’s Bureau of Ordnance decided to develop a fully automatic 3” gun mount to replace the 40mm Bofors. Initially, it was hoped that one single mount 3” gun would replace a twin 40mm mount, and similarly, a twin “3 mount would replace each quad 40mm mount. As it turned out, the weight of the new guns and mounts meant that a three-for-two exchange was the best that could be arranged. The new gun and mount was quite a bit heavier than a twin or quad 40mm mount, and a great deal heavier than the earlier simple hand loaded, hand aimed 3” mounts.

The new three inch gun differed in two main ways. First, it had an automatic loader. Second, it was controlled not from the mount, but remotely by the fire control system, either a simple Mk51 optical director, or later by more advanced radar controlled directors.

The automatic loader was basically a two sets of sprockets, one on each side of the loading tray, and the loading tray itself, which automatically rammed home the ammunition. A further advance over earlier guns was an automatic breechblock. As soon as the rim of the cartridge passed the extractors,  it tripped a spring loaded catch that allowed the breechblock to snap into place, readying the gun for firing. If the triggers were held closed, the gun would automatically fire as soon as the breech closed. Similarly, some of the mechanical force of the recoil was used to force open the breechblock during the recoil stroke, and eject the  shell case. As soon as the case had cleared the loading tray, the automatic loader would position the next round and the rammer would start the firing cycle again. This allowed a much higher rate of fire than manual operation of the breechblock. The autoloader held ten rounds ready to fire, and was set up so that as rounds were fired, the crew could continuously feed more rounds into it.


The second big change was that the mount was powered in elevation and bearing. Varying DC current inputs to the traverse and elevation motors determined the rate of change for both. The gunfire director would generate a firing solution and transmit this to the mount. The pointer and trainer on the mount were there strictly as back up in case director control was lost. Several mounts could be slaved to the control of a single director, massing their fires onto a single target, and increasing the likelihood of success.


Rather than trying to integrate a mechanical fuse setter, the advent of the proximity fuse meant that all anti-aircraft fire from these 3”/50 mounts could dispense with mechanical time fuses and simply fire VT fused rounds. Impact fused rounds were available for use against surface targets.

There were a couple different variants of the 3”/50. There were open single and twin mounts, and there were single and twin mounts with a fiberglass spray shield to protect the gun crew from foul weather. This shield offered no ballistic protection from enemy fire. Also, later alterations of the mounts often included a small radar on the gun mount to feed range information to the fire control director.

While the 3”/50 was an improvement over the 40mm gun it was clear almost as soon as it entered service that the days of the gun as a primary anti-aircraft weapon were numbered. Ever increasing aircraft speeds, and  the coming proliferation of anti-ship missiles meant guns offered only the barest protection from aerial threats. Attempts to build a completely new gun system, the 3”/70 gun, will fully automatic loading, from the magazine to the breech, and with higher velocity and greater ranger were a disaster. The 3”/70 mount was so complicated it jammed nearly every time it fired. Further, its performance was clearly not enough of an improvement to enable it to reliably defend against high speed aircraft.  For lack of a reliable, cost effective short range system, the 3”/50 remained the primary defense for many  ships in the US Navy all the way until the early 1980s.

Eventually, the 3”/50 would be phased out of service and replaced by the Sea Sparrow short range missile system, and later the Phalanx Close in Weapon System, and the RIM-116 Rolling Airframe Missile.

2 thoughts on “The 3”/50 gun- Part 2”

  1. Thank you, Brad, for this set of posts. I had always wondered if a new gun was involved, or if it was a new mount with automatic feed, reusing the preexisting 3″/50s in stock. Thank you clearing this up.

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