the Bird rifles are very rare and you see them only from time to time. I would say most of them are chambered for the 40 mm long brass cases the so called Kugelhülsen

heavy Mauser Star, Bird or Target Rifle in 20,16,24, 28 or 32 ga

heavy mauser target rifle, so called Bird Rifle in 16 and 20ga

the rifle was shoot from a simple rest on the Adlerscheibe/eagle target

http://www.google.de/imgres?q=adler+schi...&ved=1t:429,r:55,s:0,i:253&tx=46&ty=60

http://www.wittenberger-schuetzengilde.de/images/20070414173706.jpg