Complex sentence: Development of the gun began in 1936, with the contract being awarded to Rheinmetall Borsig, the first prototype gun was delivered for testing in late 1937 and completed testing successfully.
Split into simpler sentences: Development of the gun began in 1936, with the contract being awarded to Rheinmetall Borsig. The first prototype gun was delivered for testing in late 1937 and completed testing successfully.