Can I use ac MCB in dc circuit?

Don't know if its Acceptable or OK, but I used to work with an Old Army Technician, and he had a 20 or 30 Amp Square D QO Type single pole breaker for protection coming off a 12 Volt Battery for Radio Testing. Never known of any problems.
However, DC welds/ AC Breaks.
He said Amps is Amps?

The AC breaker will need to be rated to handle DC. Check the manufacturers datasheet.