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German Balkenkreuz

Germany's 2.8-cm schwere Panzerbüschse 41
2.8-cm sPzB 41

It was a taper-bore gun went from 28 mm to 20 mm. It was based on the Gerlich principle. The rounds would have special flanges that would fold back as the shell went down the barrel. This would increase the shells velocity out the end of the barrel.

There was one design, that had a lighter carriage, that was intended to be used in the airborne forces.

  2.8-cm sPzB 41
Starting Caliber 1.1"
28 mm
Emerging Caliber 0.787"
20 mm
Length of gun 5' 7"
1.7 m
Length of bore  
Rifling  
Length of rifling  
Weight traveling  
Weight in action 492 lb
223 kg
Elevation -5° to +45°
Traverse 90°
Muzzle Velocity AP: 4,593'/sec
AP: 1,400 m/sec
Range of shell  
Shell weight AP: 0.27 lb
AP: 0.124 kg
Armor penetration AP: 2.205" @ 400 yards
AP: 56 mm @ 365 m
Breech mechanism  

Sources:

  1. The Encyclopedia of Weapons of World War II, 1998, Chris Bishop
20th Century American Military History Crucial Site