Selco, interestingly, my combat sidearm of choice is also a Glock 21. I selected it because it meets my requirement for ubiquitous ammo (at least in the USA), simple gun, and stopping power. I use the model 30 for every day concealed carry. In a hot war I might move to the model 17 because stopping power becomes less of an issue than firepower. If you’re returning to base every day it’s OK to carry fewer heavier rounds but if I’m going to be away from supply for some time, I prefer a lot of small rounds to shoot rather than running out of a few big rounds. Hey, something is much better than nothing. One of my gunfighting schools has the motto “Any gun will do if you will do!” so I’m not too picky and use what’s available as well as possible.
As for rifle, in the jungle the M16 is best if used with the M193 ammo (55 gr lead core, FMJ). It blows out chunks of meat and is inhumane but, hey, the other guy is trying to kill you so who cares? The 62 gr steel tipped M855/SS109 is terrible because it goes right through nice and neatly. The bad guys keep fighting, especially if they’re on some drug, and maybe die hours later. For non-jungle I like the .308 but you can carry much less of it. The 7.62×39 is a good compromise where longer range is common. However, use soft points because the FMJ rounds will go right through again. You need at least 3000 ft/sec of velocity for a FMJ lead round to break up and the 7.62×39 only hits about 2,400.
Shotguns, *laugh* not very useful except for very close up fighting. In that case, attach a bayonet and use slugs to give yourself some reach. Slugs are great knock down power too. If you’re going to use shot, use #4 shot not 00 buckshot. 00 buckshot goes through and you have fewer pellets. The #4 keeps many more of the pellets in the body for fully energy transfer, and more pellets for better spread coverage.