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120 mm HESH vs 125 mm HE

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Proxy wars during the Cold War weren't just a one way shipment of resources. For example, the USSR managed to obtain a functional Chieftain tank in Iran, allowing them to perform thorough trials of its various components. One of the more interesting ones was the evaluation of the L31A7 HESH shell. The Russian name for it was "armour piercing-high explosive", not to be confused with the Western style APHE.

Analysis of the L31A7 shell was evaluated against homogenous and composite armour. Effectiveness of the armour piercing function was evaluated through the frequency of the oscillations that were caused after detonation using sensors mounted on tank armour. Shells were also fired against 160 mm thick homogeneous armour at 60 degrees to measure the spalling effect.

The impact against a tank's armour was judged to be similar to the effect of 115 and 125 mm HE shells: 15% less effective than the impact of a 125 mm HE shell, but 5-10% more effective than the impact of a 115 mm HE shell. The impact against homogeneous armour was judged to be similar to the effect of 100, 115, and 125 mm HE shells. 

Acceleration of the tank's upper front plate at certain frequencies on impact from HESH and HE shells.
1. 120 mm HESH at 645 m/s 2. 125 mm HE at 850 m/s 3. 115 mm HE at 780 m/s 4. Effect of 125 mm HE against the upper front hull armour of a Chieftain tank (calculated).

Acceleration of the tank's turret at certain frequencies on impact from HESH and HE shells at angles of 23-30 degrees.
1. 120 mm HESH at 645 m/s 2. 125 mm HE at 850 m/s 3. 115 mm HE at 780 m/s


Performance of HE and HESH rounds against homogeneous armour, where
J = impact acceleration
K = J/J1, where J1 is the acceleration for 120 mm HESH
Ammunition

Impact velocity (m/s)

J (m/s²)

Impact frequency (Hz)

K

120 mm HESH

646

2300

95

1.0



670

50


125 mm HE

850

2800

95

1.2



780

50


115 mm HE

780

2400

95

1.0



700

50


100 mm HE

890

2050

85

0.9



580

50



Results of effectiveness of the 120 mm HESH shell based on spalling established that 150 mm homogeneous armour at 60 degrees is the maximum where spalling occurs. Damage to the armour consists of a 10 mm dent, 2.2 calibers in diameter.

Plate thickness (mm)

Plate hardness (HRC)

Mass (kg)

Impact velocity (m/s)

Damage

Spall velocity (m/s)

Spall fragment mass

Direction of spall fragments from normal

160

3.65-3.7

17.8

645.8

No spalling. 30 mm deep bump, 240 mm, 130 mm crack. 350x260 mm ellipsoid dent on the front (280x260x10 mm formed by HE)

-

-

-

150

3.6-3.65

17.2

653.2

250x285 mm spall. Maximum spall depth: 30 mm. 390x270 mm dent on the front (290x270x10 mm from HE)

75

-

30°

140

3.55

17.16


200x190 mm spall, 20-25 mm thick. 380x260 mm dent on the front (290x260x10 mm dent from HE)

36

5.3

43°


Conclusion:
  1. The beyond armour effects of the 120 mm HESH shell from the Chieftain Mk.5P on composite armour of a tank is approximately equivalent to that of the 125 mm HE shell.
  2. When striking a 150 mm thick homogeneous armour plate at 60 degrees from normal, the 120 mm HESH shell from the Chieftain Mk.5P causes spalling that can damage personnel and equipment inside the tank.

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