It takes 4 (and a bit) to kill mediums with Puma HEAT shells judging by testing, but they do practically guarantee penetration at max range. Puma is a really strong stopgap with HEAT shells.
This are the values of the Puma
Penetration near 160
Penetration mid 120
Penetration far 80
Damage 120
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Accuracy near 0.05
Accuracy mid 0.0375
Accuracy far 0.025
This should the values with heat shot
Penetration near 208
Penetration mid 156
Penetration far 104
Damage 156
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Accuracy near 0.05
Accuracy mid 0.0375
Accuracy far 0.025
The chance to penetrate a Cromwell at max range is not 100% but 65%. In addition the chance to score a natural hit hit that range is also low (55%) bring the total chance to hit the penetrate at max range to down to 36% (a bit higher due to collision hits).
Finally there is no not 4 and something, it need 5 shot that hit the penetrate and it the chance to do so before the ability runs out make it very improbable.
The ability is not bad on puma but hardy as powerful as described.