105mm OCC-105-F1 / Obus G / Gestner shell / CTG Nt87 HEAT-T.

[pzgr40]

Cutaway model of a ‘OCC 105 F1’ shaped charge anti-tank shell. The shell is also known under the names ‘Obus G’ , Gestner shell , and the Dutch type classification ‘CTG Nr 87 HEAT/T ’.

In the 50’s of the last century tank design changed; instead of producing heavier tanks , one started to design light, fast and agile tanks with a powerful main gun. As the shaped charge was able to penetrate all armour and advanced armour was still in its infant stage of development this seemed to be the logical choice. The French AMX-13 light tank was an examples of this design.
Therefore the original plan was to only use shaped charge projectiles aboard these tanks as available anti-tank shells.
The AMX -13 used the M-57 rifled anti-tank gun. Rifled guns however have one big disadvantage however when firing shaped charge projectiles ; the spin will cause spalling to the jet of the shaped charge , diminishing the shaped charge effect with 10 to 20%. Therefore smoothbore guns (non- rotating projectiles) are preferable when firing shaped charge projectiles.

As a result of this, the unique OCC 105 F1 projectile was designed; the outer shell rotates to stabilize the projectile during its flight, the shaped charge on the inside of the projectile rotates slowly (20 to 30 rotations /min).

The projectile’s outer body consist of an upper and lower part screwed together, both housing a ball bearing racing. The inner projectile consist of a steel explosives container filled with Hexolite (a mixture of Hexogen and TNT) with a bell shaped red copper cone in the centre. To the base of the explosives container the steel fuze housing is screwed, housing the base fuze 70 with the booster above it. Into the base of the fuze housing the tracer element is screwed. A cone with a pipe piece is screwed to the top of the explosives container . On top of this pipe the piezo electric fuze is screwed.
A sheet metal cone is placed on the inside of the cone to act as an electric lead. This cone contacts the red copper cone but both are insulated from the explosives container by means of insulting plastic rings. The piezo electric fuze screwed to the top of the pipe consists of a disc of piezo electric crystals between the steel nose piece and the brass connector pin protruding in the sheet metal cone. This assembly is cast in resign to fixate it.
The outer skin of the pieze electric fuze is separated from the steel nose piece.
All together this assembly forms a closed electric loop over the base fuze , interrupted at the nose by the space between the outer skin and the nose piece, in the base fuze by the out of line rotor.
After firing of the projectile the rotor in the base fuze rotates in line, closing the circuit over the electric detonator. Upon impact the outer skin is forced inward, pressing against the nose piece (closing the contact), which -on its turn- crushes the Piezo electric disc below it, delivering an electric current, which detonates the base fuze detonator, exploding the shaped charge.

In rest, the inner projectile is connected to the outer body by means of a shear pin which breaks after firing. In the base of the projectile body radial holes are drilled that allow gasses of the burning powder charge to enter the projectile body. The gasses leave the projectile body through the longitudal holes around the upper bearing. This mechanism is a levelling system allowing the inner projectile to ‘’float freely” in the outer body. Upon firing , inertia will move the inner projectile backward compared to the outer projectile body. Pressing in the gasses will nullify this movement, allowing the inner projectile to “float” in the outer body, allowing the bearing to keep the inner projectile non rotating.
It must be noted that this projectile is unique in its design and was only used in this type of shell.
The reason the shells were used only quite shortly were the very high production costs and fast developments in armour protection that rendered the projectile useless.

This shell was used in the former AMX-13 reconnaissance tanks of the Dutch army, hence the Dutch markings on the projectile. The Dutch army used 131 AMX-13 tanks up to 1983.

Length of the complete cartridge : 905mm
Length of the projectile : 465mm
Diameter of projectile : 105mm
Penetration : 360 mm steel at zero degrees, 150mm at 60 degrees. Concrete 700 to 1000mm.
Cartridge weight : 17,60 kg
Projectile weight : 10,85 kg
Vo : 800 mtrs/sec
Effective range : 1900 mtrs

I am very happy to have finally found this rare shell I’ve been looking for for over two decades.
Thanks for the help in this matter to Erik and Kit.

Regards, DJH

[apfsds]

So incredibly beautiful! Another epic job!


Jason

[pzgr40]

Thanks Jason. Allways nice to hear.

[M8owner]

I am sorry I did not notice this post when you first published it. Outstanding work as always Jaap. I spent ten years as a mechanical engineer for a ball bearing company, and this ball bearing system is brilliant. Some engineer really knew what he was doing when he designed it. It looks simple, but it is not normal at all. The ability of the inner projectile to move up and down 7-12 mm is key to its proper functioning. Normal ball bearings would be destroyed by the firing forces. The inner projectile sits on the outer projectile when firing, and this keeps the bearings from coming apart. The use of the firing gases to lift the inner projectile to allow it to "float" and for the bearings to function is another brilliant, out-of-the-box, engineering solution. I love seeing great engineering at work. Thank you.