Comment by troad

It's not 'calm' that differs, it's 'common'. Calm like palm, in all major accents.

Traditionally, calm and com- have different vowels in English, but most North American accents merge com- into calm. All other major English accents retain the distinction.

If you're American, try saying 'com' while rounding your lips. Or just listen to a recording of 'common' in an online dictionary from Britain or Australia. (Or lot, pot, spot, etc.)

TLDR (simplified):

US/Ca: (lot = palm) ≠ start

UK/Au: lot ≠ (palm = start)