GMC T68 - 40mm Gun Motor Carriage.1942

[klem]

Designed in 1942 by the American Ordnance Company, an old-established gun-making concern of Bridgeport, Connecticut, the M3 half-track carrier, known as the 40mm Gun Motor Carriage T68 (GMC T68) and featured unusual gun layout. The guns were placed, one above the other, a configuration with an overhead equilibrator cylinder. In June 1943 the only copy of the GMC T68 was tested by the Antiaircraft Artillery Board, without success which ended this experience.(The Bofors Gun-Terry Gander)

 

[ImportantSimone5]

Designed in 1942 by the American Ordnance Company, an old-established gun-making concern of Bridgeport, Connecticut, the M3 half-track carrier, known as the 40mm Gun Motor Carriage T68 (GMC T68) and featured unusual gun layout. The guns were placed, one above the other, a configuration with an overhead equilibrator cylinder. In June 1943 the only copy of the GMC T68 was tested by the Antiaircraft Artillery Board, without success which ended this experience.(The Bofors Gun-Terry Gander)

It looks very strange.

 

[JWilly]

If you want to minimize dispersion-causing mount flex due to recoil force and vibration, don't mount the guns on substantially long levers extending away from the structural frame.

If you want to maximize vehicle stability when driving and when emplaced and firing, mount the guns as low as possible so that the vertical distance from the ground to the center of mass is as small a fraction of the vehicle ground-contact width as can be achieved.

 

[Maxrobot1]

I imagine that getting the 40mm Bofors clips fed would be difficult.

 

[JWilly]

In the early days of US Bofors adoption, there was talk in USA of re-designing the gun to normally be mounted on its side so that shell entry would be from the side with ejection from the opposite side, and to accept belt fed ammo...akin to the British Vickers pom-pom family with the same caliber. My guess is that the prototype shown above, if it had been better received, might have been re-designed for the next prototype to have the guns mounted and fed that way, once such guns existed, of course.

We think that side-by-side guns are "natural"' akin to the M42 and similar designs, but in fact an over-under design would be more sensible for side-fed, belt-fed guns.

Perhaps a bit of hubris on the part of American Ordnance Company. Certainly not the first time that American designers have assumed that weapons and vehicles designed outside USA must be inferior and would benefit from an American re-design.

 

[Scott Kenny]

In the early days of US Bofors adoption, there was talk in USA of re-designing the gun to normally be mounted on its side so that shell entry would be from the side with ejection from the opposite side, and to accept belt fed ammo...akin to the British Vickers pom-pom family with the same caliber. My guess is that the prototype shown above, if it had been better received, might have been re-designed for the next prototype to have the guns mounted and fed that way, once such guns existed, of course.

We think that side-by-side guns are "natural"' akin to the M42 and similar designs, but in fact an over-under design would be more sensible for side-fed, belt-fed guns.

Adding a belt feed would have been a great improvement to the Bofors 40mm L60. It would have allowed much fewer crew per gun tub.

Perhaps a bit of hubris on the part of American Ordnance Company. Certainly not the first time that American designers have assumed that weapons and vehicles designed outside USA must be inferior and would benefit from an American re-design.

Considering that Ordnance Branch went through one set of blueprints and changed dimensions from (example) 5"-0.010" to 4.95" +-0.005" JUST TO CHANGE TO PLUS OR MINUS, I can't argue with that. Note that the weapons made to the changed prints did not function at all.

 

[Petrus]

In the early days of US Bofors adoption, there was talk in USA of re-designing the gun to normally be mounted on its side so that shell entry would be from the side with ejection from the opposite side, and to accept belt fed ammo...akin to the British Vickers pom-pom family with the same caliber.

Sounds very interesting. Where may I find more information on that?

 

[JWilly]

I saw that described years ago, and I'm not sure where. I thought it was in the extensive discussion at NavWeaps (http://www.navweaps.com/Weapons/WNUS_4cm-56_mk12.php), but I've read through all of that again and at present it's not there. Maybe in the Gander book? I no longer have my copy.

However, my recollection is that the side-belt-feed weapon version was actually developed even though not adopted, and it was this weapon version that was resurrected in the 1980s by USA as the Mod 9 version of the gun, which then was equipped with a (massive) belt drum (think the Oerlikon 20mm 60 round drums, but much larger), which mounted to one side of the gun. I believe the drum was loaded with a continuous belt, then the drum-drive was adjusted to the setting that would just balance the round/belt friction so that the gun star wheels could pull rounds out of the drum with no more effort than was required to pull rounds vertically from the four round clips. My understanding is that the 40mm drum was a pull design, requiring belted ammo...not a push design that could be filled with loose rounds that would be pushed around the spiral track by the drum drive.

These 48-round-drum-fed Bofors guns were used as bow weapons for a while on USN PB Mark III "Specter" power boats. They were intended at the time to reliably feed the 48 rounds at up to 120 rounds per minute even under highly dynamic boat speed/water-and-weather roughness conditions, which human loaders were thought to be incapable of matching.