Michael Mack’s poem ‘Small Pain in My Chest’ is rich in its use of figure of speech. Here I list a number of them which I’ve found out so far.

Alliteration
Alliteration is repetition of consonant sounds in the beginning of nearby words.

What would my Wife be thinking…

Again, there is Sibilance, a type of alliteration in a few places. It is mainly a repetition of ‘s’ producing a hissing sound.

She could see me sitting here
I see the sun is shining

Irony
I think, there is a certain irony in the expression ‘a small pain in my chest’, as we see that the wound was actually a fatal one and the soldier boy finally succumbed to it. The poet might be hinting that the injury of the soldier boy was not a great injury compared to the destruction war can cause and the intensity of pain the death of the soldier boy could inflict on a right-thinking man like our speaker.

Repetition
The poet has repeated the expression ‘a small pain in my chest’ at the end of each stanza beginning from the second one. This works as a refrain, a regular feature of a ballad, a poem meant to be sung.

Symbolism

Can it be getting dark so soon?…
I thought that the day had just begun.

These two lines are symbolic. Here, ‘getting dark’ signifies the approaching death of the young soldier. ‘The day’ indicates his life which he thought had just begun.

Polysyndeton
It is a repetition of conjunction.

The battle had been long and hard and lasted through the night

Euphemism
This is the use of good-sounding indirect words to substitute harsh and unpleasant ones.

They’re all gone while I felt this…

In the above example, ‘gone’ is used to mean ‘dead’ to make it sound milder.

Hypallage
Hypallage is an interchange of two elements in a phrase or sentence from a more logical to a less logical relationship, as Merriam-Webster defines it.

The night exploded and …

Here, the night didn’t really explode, rather the bombs exploded in that night. So, it’s an instance of hypallage or transferred epithet.

Adnomination
It is the use of words with same root.

And smiled a smile …

Here, ‘smiled’ is a verb and ‘smile’ is a noun. The two words are originated from the same root ‘smile’.

Hyperbole
It is an exaggerated statement, generally to make an emphasis.

…the brightest that I’ve seen.

The speaker has described the soldier’s smile as the brightest smile he has ever seen. Actually it depends on the mood and feeling of a person in a given situation that he finds something too good or too bad at different times. Clearly this is an exaggeration.

[Now, as a reader, if you found something else, please submit an answer below and help others.]