"R" sounds only as "R", not "Ş". There is only one thing about "R" you should keep in your mind. This is not a rule but we almost never pronounce "R" when it's at the end of a verb as present continuous tense particle "-(i)yor but it's pronounced if there is a vowel next to it. You might hear it's being pronounced only in songs. Example: to come -> gelmek (verb stem is "gel") (I)'m coming -> (ben) geliyorum (pronounced as it is) (You)'re coming -> (sen) geliyorsun (pronounced as "geliyosun") (He/She/It)'s coming ->(o) geliyor (pronounced as "geliyo") (We)'re coming -> (biz) geliyoruz (pronounced as it is) (You-plural-) coming -> (siz) geliyorsunuz (pronounced as "geliyosunuz") (They)'re coming -> onlar geliyorlar / onlar geliyor (pronounced as geliyolar/geliyo)

I had this issue as well when I first started Turkish. I asked my Turkish friends why they said "ş" instead of "r" at the ends of certain words. They gave me a strange look and said "We don't." ಠ_ಠ It turns out the final "r" in Turkish is very soft, but to Turkish ears it doesn't sound like "ş".

If you give an example maybe we can understand better. In phonology Turkish R is alveolar tap, its a little stronger than English postalveolar approximants but nowhere near a trill like Russian or Spanish. Maybe thats why. I have same thing in another language. Japanese R sounds between R and L to me but i can't pronounce it. Turkish R is usually lost at ("-iyor"), roots don't lose their R that easily. This is loss more prominent in local dialects not in standart language. It is becoming more common though. Even in Istanbul (where standart language is based of) about half of the people would drop R. After i searched a bit it is a Final Obstruent Devoicing (Consonant Devoicing) case. But i couldn't find details of it. Edit: I failed miserably there is no such thing as kaynaştırma harfi olan R. I meant the present simple R