Krupp built a test model in late 1939 and sent it to the Hillersleben proving ground for testing. Penetration was tested on this occasion. Firing at high elevation, the 7.1 tonne shell was able to penetrate the specified seven metres of concrete and the one metre armour plate. When the tests were completed in mid-1940 the complex carriage was further developed. Alfried Krupp, after whose father the gun was named, personally hosted Hitler at the Rügenwalde Proving Ground during the formal acceptance trials of the Gustav Gun in early 1941.