First Time This Happened To Me...

[goodlookin1]

Was seating a primer in a Lee Turret Classic press in a 300 Blackout case. Seating went fine with no abnormal function, but on the last little millimeter of insertion, the primer blew on me! It was one smooth motion, not a jarring insertion.

I inspected the primer seating assembly and the primer itself: Nothing sticking out on the face of the seating arm, and no dent on the primer itself.

What's the likely cause of this? Was it avoidable, or just a fluke?

Remington 6-1/2 small rifle primer.

[Divernhunter]

Probably will never happen again.
I have been loading for many years and have not had that happen.
Then I stopped priming on my press and hand prime now---Except for when I use my Dillon650 for 45ACP and 9mm Luger.

You will find an improvement in accuracy with hand priming.

[pacrat]

I started rolling my own ammo in 1968. I've never popped a primer during the process. Even though I've deprimed a lot of cases with live primers for reuse.

Congrats OP, you won the unlucky lotto.

[HTLP]

I have crushed a few primers in my press but never had one go off, however I have never used Remington primers. I almost exclusively use Winchester primers but I occasionally use CCI too. I was told when I started reloading that Remington primers have thin cups and can be a contributor to slam fires. Maybe your primer had a thinner cup than usual.

[Eljay]

I never had one blow on my turret press (10-15K rounds). I have had one pop on my 650 (20K rounds) which was a real wakeup. I had recently switched to Federal primers and I had some brass that was NATO or similar and still had a bit of a crimp and I forced it and BAM! Winchester or CCI would have just gone on in. So after that I now stop at the first hint of a problem and depending on the caliber either just throw away the brass (if I have a ton and it's stuff you can find laying around) or inspect/swage it (if it's a caliber I have limited supplies of).

I've never used Remington primers but if they're similar to Federal I'd just say be gentle with them.

[stilly]

Was seating a primer in a Lee Turret Classic press in a 300 Blackout case. Seating went fine with no abnormal function, but on the last little millimeter of insertion, the primer blew on me! It was one smooth motion, not a jarring insertion.

I inspected the primer seating assembly and the primer itself: Nothing sticking out on the face of the seating arm, and no dent on the primer itself.

What's the likely cause of this? Was it avoidable, or just a fluke?

Remington 6-1/2 small rifle primer.

Says it all...

If you drop their guns they will fire (safety does not matter) if you prime with their primers I guess you get this. I dunno, but I have never had any such incidents with any Tula, Wolf, S&B, or Winchester. Even CCI have not given me issues. But if the metal is softer than others, that could explain it.