The Cambridge Dictionary transcription for the word barn is /bɑːrn/
If someone says this word as /baːrn/ (open front vowel), will this sound foreign to you? Will you notice at all? What will your thoughts be on the speech of a person who changes his /ɑ/'s with /a/'s in words like this?
I'm not sure what I'm doing but here's me saying /bɑːrn/. and then /baːrn/ : Vocaroo speech example
Can you try it yourself as native speakers?