Dud Primers?
 

Yesterday I had 5 out of 14 rounds not go off. Primers never fired. Huge dent in the firing pin. This is the first time in 20 years that I had any duds. They were Federal Large Rifle Magnum primers in a 300 Weatherby. The only thing I did different was I sized and primed the cases months ago. I just for around the loading them Friday. Is it possible that they got moisture from sitting in the cases for 6 months? I mean, in the package, they're not really sealed. I can't think of any other reason for such an occurrence. Buck season opens in a week.

I often keep sized and primed cases in covered plastic containers for months and never had a primer failure. Primers are pretty resistant to moisture. About the only thing(s) that can make them fail are oil or damage to the anvil.

I would get a new box of primers and tear down the loads and redo them.
I used spry lube once and it had got into the inside of the cases and seeped down onto the primers. Never used it again !

You must not have used Hornady One Shot. That stuff is great and will NOT contaminate primers or powder.
I've been using it since it first came out without a single failure.

How's your seating depth on the primers? Even with contamination, that's too many failures. More likely they weren't seated deeply enough, and the firing pin had to finish the job, failing to give a sufficiently rapid crush against the anvil to ignite the primer.

Another good point. I didn't even think of that.

tear your bolt down and clean it
RR

Second this too.

don,t use wd40 near ammo, it can seep into primers if you spray cases or loaded ammo, Ive seen guys spray loaded cartridges to reduce brass tarnishing, and the result was a few mis-or hang fires a few weeks later when that ammo was used.