First of all I am a serious 1911 fan and not an expert on the HP but I'll try to fill you in and hope someone with more knowledge will help. The Browning HP was invented after the 1911 and many see it as J. M. Browning's attempt to "improve" on the 1911. It is a fine design, using the same locking mechanism, and it has a double stack magazine that holds a LOT of 9MM rounds. Certainly it was one of the first "high capacity" 9MMs. Browning tried to sell it to Colt but they didn't want it so he went to FN in Belgium who also had a history of buying his designs. I believe the European designation for it was "P35" but it was marketed in the US as the High Power.
You could have your hero buy a 1911 when in the US and wish he had brought it back to Germany with him. Then discover the similarity of the P35 and the advantage of many more rounds (a full magazine and two extras in a belt pouch would be a lot of firepower in those days for a pistol). Better acceptance by his fellow officers as well.
The full jacketed 9MM as required for military use by the Geneva Convention is an excellent sub machine gun round but not a good "one round" man stopper. Modern hollow points are much better. Allied solders were known to alter ammunition to achieve expansion in WW2 but if captured by the Germans with such ammo, they were badly treated, even executed. Such altered bullets were often called "dum dum bullets" (possible misuse of the term) and the damage claims often gruesomely exaggerated. Your hero could agonize over the morality of using such bullets but justify it by admitting he would be saving the last one for himself as capture was not an option.