Oh yeah, ain't that fun. Well, on the bright side, if you find the missing cup in a round already loaded you'll know if you flinch or not

Lol now this is a glass half full kinda guy

Well, I searched and scanned each primer after that, and all of them had an anvil. So, all looked good. But when I saw the loose anvil, I was like craaaaaapppp!

The new style ‘lite’ primer.

Ruh Roh!

That just means you already loaded it. Definitely a flinch test coming up for someone

It was a new sleeve of 100 count primers. I checked each primer, and all looked good. So I think it's just a loose anvil in there.

I sure hope there's not a "click"! That went in some 375 Raptor ammo, and it ain't cheap!!

Well never had an extra anvil in a box of primers but have had an anvil seated backwards and caught it flipping primers

Well after loading thousands and thousands of pistol rounds from about 1990 or so until six years ago, shame on me for asking!

What is the risk here? Is there a video on the anatomy of a primer I need to watch?

In all that time I had one primer go BANG in my face using the Dillon 550. It sounded about like a 22LR. To this day I don't know if the primer was sideways, improperly manufactured or what. I just know my Tinnitus didn't like it at all. Other than extreme noise that was the damage.

Here I'm about to start loading again and figure I'll be wearing safety glasses from now on.

So to the original question, do you have to "S can" a pile of expensive ammo?

If you have a loose anvil in a box of primers, it could have fallen out of the primer and would make that primer a dud. So "no go bangy!" I inspected all 100 of the primers and saw no primer missing an anvil. So, it tells me the primers are fine and there just happens to be an extra anvil in the package. At least that's what I'm hoping for!

So no personal physical risk--right? So now there could be a click on a nice animal??

Well I guess not if you were able to verify all the primers.

Basically, yes.

On your situation, with the primer popping and blowing back, there's many reasons that could happen. If a primer catches an edge and seats off center or sideways, it can certainly pop and blow back at you. The flash hole could also be clogged or blocked. But if you load enough ammo and shoot enough, you will see some rounds have a variety of issues.

I've seen a normal match round with a Fed 210M primer have a solid hit from the firing pin, and not go off. My shooting partner ejected this live round right under my face during a shooting stage which I did not like, and we laughed about later. Upon inspection, all parts look fine and is a mystery why it didn't go off.

Yeah I've read stories on other gun forums about duds, and wondering if it was a firing pin issue. I wonder how long they waited, or how they ejected them? I've had a whole lot of bulk pack Remington 22 LR duds. When that happened I just held the gun way out, with the port facing out, and ejected them.