Different strokes, different folks. Different guns, different countries. Here's a map showing the military issue rifle for each country in the world. You'll see familiar M4s, AK-47s, AK-74s, some M16s and tinkles of other weaponry. Judging by the map's choice of colors, it seems to group weapons that are of a similar family together by color.
To be sure, there are more guns used by the military than just these but this map gives a general gist of each military's standardization. Here's a rundown of what some of these guns look like, courtesy of Wikipedia:
Heckler & Koch G36
You can hit the expand button for a bigger map. Full-sized version of the map can be found here.