The opening phases of Operation Barbarossa, the German  invasion of the Soviet Union in 1941, brought a large quantity of captured war materiel into into the German armoury. So many Soviet anti-aircraft guns were captured and were taken into the German Inventory. When all these captured guns were first issued they continued to fire stocks of Soviet captured ammunition. 

When captured stocks began to run low a decision was made in 1943 to rebore the barrels to accept standar German 8,8 cm Flak 18/36/37 ammunition. the measure was intended to simplify logistic and prolong the service life of the captured guns.

The 76 mm air defense gun M1938 Soviet air defense gun, created by Soviet artillery designer M.N.Loginov. This gun was a modernized version of the 76.2 mm air defense gun model 1931, with a slightly modernized barrel and a completely new two-axle carriage ZU-8.

For sale: German 88mm Flak ex Russian 7.62 AA gun (photography via Milweb)

These guns were used in the first stages of the German-Soviet War. The muzzle velocity of the 6.5 kg (14.33 lb) projectile was 816 m/s (2,677 ft/s) and this allowed the shell to reach a 9,250 m (30,347 ft) maximum altitude.

76 mm air defense gun M1938 (photography via Wikipedia)

The Model 1938 was also taken into German service as the 7.62 cm Flak M.38(r), and some of these were rebored to take 8.8 cm ammunition when they became the 7.62 cm/8.8 cm Flak M.38(r). They remained in use by the Germans until the end of the war, as did the guns the Russians retained. 

Data :

• Calibre: 76.2 mm ( 3 in) 
• Length of Piece (L/55): 4,191 mm (165 in) 
• Length of Rifling: 3,606 mm (141.97 in) 
• Weight Travelling: 4,210 kg (9,283 lb) 
• Weight in Action: 3,047 kg (6,718 lb) 
• Elevation: - 3 to + 82 degrees 
• Traverse: 2 x 360 degrees 
• Muzzle Velocity: 813 m/sec (2,667 ft/sec) 
• Maximum Ceiling: 9,300 m (30,500 ft) 
• Shell Weight: 6.61 kg (14.575 lb)