Before I show you the expected Sixgun combined robot mode , let me tell you something...
....my Sixgun actually has a secondary combined mode..... a jet !!! The photo above and below showed how the guns are connected to form this jet, By the way, do check out part 1 if you have not already.
Although the red gun is connected to SG-01 main weapon via merely just one stud + technic hole, such connection is quite strong.
Transforming to Robot Mode