A morning at the range with my Sauer Model 54 16ga over .30-30 revealed that:

With the non-adjustable folding rear sight flipped up, the rifle barrel sent bullets exactly where I aimed at 50 yards. However, slugs (of various brands and weights) out of the shotgun barrel flew 12-16 inches high with the sights.

Similar results (high) with shot patterns at 25 yards using the sights.

With the sight folded down the shotgun barrel shoots flat, with nicely centred patterns at 25 yards.

In other words, it shoots well as a rifle when in rifle mode and it shoots well as a shotgun when in shotgun mode, but one must not mix the two.

Presumably this is how it should be? Thanks in advance.