I'm guessing this 426AD3-4 somehow managed to slip past the heat treatment. It was bucked no more than the others and even the manufactured head got squashed!
The 3rd photo is after pushing in a centrepunch by hand.
If you got a bunch of these and set them with a pneumatic squeezer, you might not even notice, except for the squashed manufactured head.
Check the Aircraft Spruce catalog for the soft "A" rivet description. One of these could have gotten in your set. The tensile strength is lower than the "AD" rivet.
I don't see a dimple (alloy/temper identification) in the mfr'd head. Maybe it got flattened out when you shot it. Or maybe not. If there's no dimple in the head before you shoot it, it ain't an AD.
It seemed to be squashed enough to flatten the dimple if there was one. I don't know if I would have noticed an unmarked rivet or not but now I will be on the lookout for any 'A' rivets. This was a pain to remove; it would bend and squash instead of coming out cleanly.
Soft rivet
Maybe just an "A" and not an "AD"
Linear Mode
