on page 162 about "Impact" the last part of the section reads...
However, reading the PDF version, this text is missing!... (the Effect cannot exceed the number of missiles in the salvo).
Based on the PDF reading of that section, it is possible for a hit from a missile salvo of one to do: (4D - armor) x 5 damage. [roll 8+ with DM +1 for number of Missiles => 12 +1 = 13 - 8 = Effect 5]
Was this intended errata?
My gut reaction is that the limit in the printed book seems logical.