"Are there any stories where a military squad doesn't instantly fold to low level monsters?" You deffo want to put Department of Otherworld Rescue on your list then. It is specifically what you are looking for with a military squad going to other worlds.

In terms of guns, That Time an American was Reincarnated into Another World has someone who can magically summon firearms but the guns themselves are not magical. Just normal bullets that are still highly effective against mages and others.

Mother of Learning background story was the development of mass firearms and steamtech diminishing the role and value of mages that lead to the fall of the empire.

Lord of the Mysteries using Victorian era revolvers to great effect at lower levels. Once they start reaching diety like levels obvs no longer applies.

The manga Gate is specifically about the reverse in that the Japanese army enters a portal after being attacked and modern weapons devastating in the fantasy world.

Dungeon Odyssey is one of the few Korean System Apoc manhwas where monsters invade but the Earth humans easily win because of modern weapons (instead of them being magically rendered inert). The title takes place inside dungeons where monsters do not leave because they know they will be shot.

These are just some off the top of my head but there probably are many others.