One report from summer of 1942....
Report of Panzer Regiment 33, dated 31 July 1942:
‘Penetration ability of the long 75mm gun KWK 40 /L43 panzergranate 39 against the T-34: The T-34 is cleanly penetrated at every angle that it is hit at ranges up to 1.200 meters’
‘T34: The T34 that was far superior to the German Panzers up to the beginning of the Spring of 1942 is now inferior to the German long 5 cm Kw.K. L/60 and 7.5 cm Kw.K.40 L/43 tank guns. After the Russians attacked the German Panzer forces in several battles with the T34 and received heavy losses, they didn't send the T34 tank against the German Panzers so long as they had a chance to with-draw’
This report, from Panzerwar website, shows L43 penetrating out to 800 meters with Pzgr 39.
I have taken the graph and made the red line correspond to a 750M/s muzzle velocity curve. At 800 meters distance, it corresponds to ~650 M/s velocity.
The 'rot' has clearly differentiated itself from Pzgr 39 at this velocity. It has 'kneed-over' on its graph and approaches an asymptote. It would not penetrate at 800 meters if fired at a muzzle velocity of 750 M/s (assuming it has similar flight characteristics). That is, further velocity does not improve its penetration, and more than likely increases shatter. In other words, the 'rot' "never gets there".
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