The reason why people do not stop when hit by the 9 mm is because they are all hopped up on drugs, even the chineese and the Jap's did that to their troops in WW II and the Korean war.

The reason why I think that they might want to change is becuse everybody and their brother has a 9mm and if you captured their supplies, you could use their ammo against them.

The Japanese had about 4 different rounds for their 7.7 Jap rifle in WW II and none of them interchanged. The logistics were aweful, but just think of it like this, even if you captured their weapon, if you didn't have the right ammo - you still couldn't use it against the other guy.