It is chambered for the .30 Caliber M2 Ball Ammo; the same round as the M1 Garand; aka .30-06. I've only shot .30 M2 out of mine (and only 10 rounds or so - I need another range trip with it).
My wife is Colombian; so in addition to the Madsen, I have also acquired one of Century's Golani Galils - the Colombian Army still carries a lot of 5.56 Galils; in evidence with security details in front of buildings where senior government officials live and work; my step-son, doing his compulsory military service trained on one. The first time he saw mine it was like a switch flipped in his mind and he went through the manual of arms with it as if he did it daily! Next time he visits we'll go to the range and see if he remembers how to shoot it!






Enjoy!