Greetings,
Umm, even the M1 and M60A3 were vulnerable to cal .50 fire from the rear arc.
The cal .50 was the primary killing system for halftracks and other lightly armed vehicles during WWII (allowing the Main Gun ammo to be saved for better targets). Heck even today, the cal .50 is good against most APCs.
I will look and see what hard data I can find.
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Been looken. At 500m a WW II M2 API penetrated 19mm of RHS. Looks like the Pnzr IV had rear protection of 15mm, T34 40mm and Tiger 80mm. I so stand corrected! Still at 76mm APCR at 500 and 1000m was 93 and 68mm, APCBC was 56, 50, 45 (500, 1000, 1500m).
T34 frontal armor was 52mm, side was 45, rear was 45 (less than M2 penetration

. Notable, turret top was 15mm (less than 19mm at 500 meters for you diving P47 drivers!)
Regards,