I'm not sure about this, but I have a hunch that if a barrage crit (on 1st hit?) The subsequent hits will definitely scores a crit.
I couldn't find any viable explanation of while my Slug Shot is doing 3k, my crit barrage doing up to 5.4k+
It is possible the same rules of missing a shot from barrage applies to critical hits as well. As whenever the following hit misses, subsequent hits after that will always miss. So if the second hit missed, all 6 following it will miss as well, and you will only get one successful hit.
Hard to test the crit hit thing as there are many variables especially if the critical hit applies to the last or last few of the hits.