I am confused how to determine hits from a combat shotgun in Dark Heresy. How does the scatter property interact with an auto-fire action when attacking?
It's confusing but they answered it in the Errata:
The Actions section starting on page 190 should include a special note concerning combining semi-auto and full-auto ﬁre with the Scatter quality, which reads “When ﬁring a semi- or full-auto burst at point blank range with a weapon that has the Scatter quality, the extra hits for rate of ﬁre and scatter are worked out separately and both applied. For example, Horatius Kane ﬁres his combat shotgun at Heretic X. Kane is at point-blank range and ﬁres a semiautomatic burst. Kane rolls 01 with his modiﬁed Ballistic Skill of 70 (30 BS, +30 for point-blank range, +10 for ﬁring semi-auto) and hits by an amazing six degrees of success. He gets one hit at 70, one hit for semi-automatic at 50, and a third hit for semi-auto at 30 (he does not get a fourth hit at 10, because the combat shotgun’s rate of ﬁre is 3). He would get additional hits for scatter at 50, 30 and 10, for a total of 6 hits on Heretic X, most likely shredding the cultist to bits in the Emperor’s name.”