Here is the example where I found this word:
Адвокаты назвали стрелка из Колорадо душевнобольным
Shouldn't it say стрелок?
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"Стрелка́" is "стрело́к" ("shooter") in accusative.
In this sentence, "стрелка" is correct, because it is the object of "называть", which should have the accusative case:
Адвокаты назвали стрелка из Колорадо душевнобольным.
Attorneys called the shooter from Colorado a madman.
Here is how word "стрелок" changes:
The vowel "о" disappears in the suffix for historical reasons. This vowel stems from a reduced vowel "ъ" of the Old-Russian language. There were two reduced vowels in Old-Russian, "ъ" and "ь", and they disappeared in "weak" positions and changed into "о" and "e" in "strong" positions. One of the weak positions was the position before the stressed syllable, one of the strong positions was the position in the stressed syllable. This is exactly what we see in examples like "сынок":
Nom: сънъкъ → сыно́к ("ъ" under stress becomes "o")
Gen: сънъкa → сынка́ ("ъ" before the stressed syllable disappears)
Now, as @Quassnoi says, this suffix appeared in the sense of profession much later, when the reduced vowels has already disappeared. But by that time it already existed in a two forms: "ок" in Nom Sg and "к" in other positions.
That is why there are disappearing vowels (called "fleeting vowels", thanks to @Quassnoi for terminology) in Russian words (and not only in suffixes, but also in stems, as "сон"–"сна" ("dream")).
Of course, there is no way you may know whether some "o" in some word stems from a full Old-Russian vowel "o" of from a reduced Old-Russian vowel "ъ", that's why you have to learn two word stems for nouns where the stress can be both on the stem and not on the stem.
Since you've mentions suffix mutation in the title, I guess you question is about vowel missing.
The thing is in Russian the vowel suffix
-ок is reduced in during declension. In words with
-ик suffix the vowel remains.
But! There are indeed cases when word with
-ок endings is different.
игрок do not lose
о in the suffix, since this will cause a
quite untypical combination of consonants -
игрка is inappropriate from the point of view of Russian phonology.
Also, one should not confuse
ок as suffix and
ок as part of the root ot the word. Examples: