Using Articles With Acronyms

Use of the indefinite article “an” before an acronym that begins with M.?
The basic rule for using the indefinite article “an” is that the word following begins with a vowel or with a silent H.
But what if the world following is an acronym that begins with M?
The acronym is MC1 (used to describe a job category, but that’s not really relevant)
which is it?
a MC1
an MC1
I say “an MC1 “but when I write it out it looks weird.
An is correct for MC1, but would not be correct for something like ZC1 as it is the initial sound that is important not the letter.
Vowel sounds = an, others = a
ABC Extingusher