I have a question about the (updated) rules for ammunitions, especially flechette and APDS. Are the modifier cummulative, or do they replace the AP of the weapon? The Errata for the Arsenal seem to imply the latter as all shotguns and other weapons with flechette-ammunition get a modifier of +5. Were it additive, the Mossberg AM-CMDT should have an AP -1+5=+4. On the Other hand the BBB clearly states that the modifiers are additive.

Does that mean all other modifiers replace the default AP as well? So a sniper rifle with APDS gets the same AP as a light pistol?

As usual, the SR books are plagued by several editing errors. The AP modifiers are supposed to be additive.