Yes it's a .50. I agree it doesn't look like the pin is extending far enough. Yep, fired it a bunch of times, but hadn't for a while. When I set the headspacing I used an average of 4 different ammo's. None of them measured exactly the same, but they were close.
I'd forgotten about it but a couple of years ago I was getting some blowback while shooting her. Turned out the cases were cracked in quite a few of the shells in a certain batch of ammo I'd bought. Maybe that blowback was getting into the bolt assembly and fouling things up. Looks like I need to tear into her, give a good cleaning, and maybe look for wear or galling.