Due to a shortage of DShK machineguns, the T-30 and T-60 tanks were equipped with the 20 mm ShVAK autocannon. Despite the larger caliber, penetration was not much more impressive than the DShK was capable of.
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- The muzzle velocity of the 20 mm AP-I round (B-3) is 796 m/s and 892 m/s for the round with NB-200 (nitroglycerine gunpowder) propellant.
- The precision of the ShVAK gun at ranges of up to 1000 m in bursts of 2-3 shots is satisfactory.
- The TMFP sight scale designed for the 12.7 mm machinegun cannot be used for the ShVAK 20 mm cannon. A new sight scale needs to be applied for the optical and iron sights.
- Against specific targets (battalion gun, tank, running infantry) firing the 20 mm gun on the move without short stops was ineffective.
- The armour penetration of the 20 mm ShVAK, same as the 12.7 mm DShK machinegun, is ineffective. Not a single penetration was obtained when firing against 20 mm homogeneous or 20 mm cemented armour placed at 30 degrees from a range of 135 meters, and when firing at 15 mm cemented armour at 30 degrees, penetration was only achieved from a range of 100 meters (firing NB-200 ammunition with increased muzzle velocity."
Not much data, but there's something to compare with a report I posted earlier (CAMD RF 38-11355-832) Unfortunately, this document doesn't cover the 20 mm ShVAK, but does contain data on the DShK. The 12.7 mm DShK penetrates 15 mm of Czech surface hardened armour on a Pz38(t) at 20 degrees from 150 meters 4 out of 12 times and from 200 meters 7 out of 9 times.
Similarly, 20 mm plates of German armour at 20 degrees in a PzIV is penetrated by the DShK from 100 meters 2 out of 2 times.
There is also some data on penetration of the 12.7 mm machinegun in a report on the quality of German tank armour (CAMD RF 38-11355-778), which I referred to previously. A tactical diagram of "German light armoured cars and APCs" shows that the sides can be penetrated at an angle of 30 degrees from up to 250 meters and up to 400 meters at normal, but given the various different geometries of German light armoured vehicles, it's hard to make any definitive conclusions.
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Interestingly enough, the conclusions state that the data obtained in the report can be used to compose instructions for T-60 tank crews, so it seems that the penetration of the 12.7 and 20 mm guns was still considered equivalent by 1942. Neither weapon was going to be setting any records, but practical trials show that the initial pessimism was a little premature, and the T-60 was far from helpless when facing off against armoured vehicles of its own class.
Penetration data via Dmitriy Shein.
Penetration data via Dmitriy Shein.