A barrel release mechanism slid into the mag well and allowed you to open the gun’s barrel and load a single rifle round into place.

The S.A.S.S. was offered in two different barrel lengths — 10¾” or 14.9″. Both lengths were offered in .22 LR and 7mm BR. The longer barrel length was offered in .223, .243, 7mm-08, .308 and .358 Win. The shorter barrel length offered the additional chambering options of .357 Mag. and .44 Mag.

http://www.louiscandell.com/pdf/firearms/springfield_1911-a2_sass.pdf