As I understand it, a AR9 type PCC does not have that obvious feel (as pistols do) when a squib happens. Especially if the bullet traveled 10in then stopped in barrel, yet action still fully cycled. The pic above I suspect is a JP test, but I dont know that to be fact.
Mostly this is a too-low powder-charge situation as opposed to primer-only discharge. JP, I think, has suggested using steel-plate target for those first few rounds of your freshly made super-low power-factor ammo - just to be sure the bullets leave the barrel.
I have never experienced a squib in my PCC yet, 20,000+ rounds, so I dont know what it feels like to have a round stick 10" down the barrel, or if carbine will cycle.