I was talking about all double flush rivets on RV's.
There is not an expectation for a fully filled countersink.
If the recommended length rivets are used on the RV-14 (or the RV-10) flap hinge brackets, they are not long enough to fully fill the countersink.
Even as thick as the material is on the hinge brackets, it is possible to cause distortion in the material by trying to fully fill the countersink.
Hinge bracket trailing edges that are straight are much more important than having the shop head countersink fully filled with the rivet.
If you countersink to a depth on both sides that would result in a flush fitting manufactured head, and then install the specified rivet with a flush rivet set on both sides using a squeezer and squeeze the shop head flush to the surface, you have met the structural expectation for this part.
If this technique is used you will still be able to see some of the countersink around the perimeter of the shop head.