Hello Dc1
Welcome on your first posting.

Pictures would be most helpful!

Your shotgun was made in December of 1923

http://www.shotguns.se/html/germany_1890-1945.html

Most likely a 65 mm chambered gun so use 2 1/2 inch ammo.
http://www.rstshells.com/store/m/4-20-Gauge.aspx

Welcome
Mike

p.s. fun reading
http://www.dogsanddoubles.com/2013/06/good-gun-alert-20-gauge-charles-daly-diamond-grade-double/