We have all participated in debates about how harmful oils are to primers.

In fact, I, along with most folks, often said, "I bet WD-40 will quickly kill a primer. You had better keep it away from your ammo."

But, just how fast does this happen?

I had one respected firearms instructor tell me that in the post I did on Cleaning and Lubing a Semi Auto Pistol, I needed to warn folks not to get any oil in the firing pin channel.

He stated that any oil on the firing pin or spring or channel would drip down on any loaded round in the chamber and quickly kill the primer.

I told him that I had been cleaning pistols like that for almost 50 years and had never had a problem. But it got me worrying… what if this was true?

I recently saw an article where a guy tested this phenomenon of oil and primers and decided to do my own test.

First, let's settle the question: "Will penetrating oil "kill" primers?"

I placed a few Small Pistol Primers in a dish.

39-1Then I sprayed them with WD-40 and let them soak for several hours.

39-2I then seated them in a sized 9mm case...


... and loaded them, one at a time, in my Beretta 92FS.

I pulled the trigger on them and they all failed to "pop".

They were dead. Here's a picture of the dents in the primer made by the pistol.

Good, hard strikes, but no joy.

There is no question that penetrating oil will "kill" a primer, if sprayed into the open end of the primer.

But how would it affect loaded ammo?


Here's what I did.

I placed separated lots of 10 rounds of Winchester 9mm White Box ammo in a holder.

I choose 5 different oils to test.

I then placed a "puddle" of each oil on the primers of the rounds and let them sit, undisturbed.

I tested them with the following oils:

Birchwood Casey Synthetic Gun Oil, Break Free CLP, Kroil Penetrating Oil, Hoppe's #9 Bore Solvent and WD-40


Here I am putting the oils on the primers.

Here I am putting the oils on the primers.​

Here's how they looked with the oils puddled on the primers.

Here's how they looked with the oils puddled on the primers.​

And here.

And here.​

After I puddled the WD-40 on the primers, Tman said, "Someone will say that if the WD-40 was sprayed with an aerosol can, it would make a difference."

So, I set up another set of cartridges and sprayed them with WD-40 out of an aerosol can.

Now everybody will be happy.

I decided to test them as follows:


I will shoot one round of each group, each week, for 5 weeks.
Then 5 shots each on the 6th week.

We would look for failures to fire or "weak" shots as evidence of primer deterioration.

Week One:
After one week, the rounds still had puddles of oil on the primers.

It was interesting that the Hoppe's #9 had turned the brass green.

Pretty neat, isn't it?


We went to the range after one week. I used my Beretta 92 as the test pistol. I loaded one round in the magazine and shot it. If the slide failed to lock back, we would consider that a sign of a "weak round".

I aimed at different place on the target for each shot, just to be sure I got a hole in the target, to assure that I didn't leave one in the barrel. Long story short…..every round shot just fine and seemed to be at full power. Who'd a thunk it?

Week Two:
All rounds fired just fine.
Normal recoil and ejection.

Week Three:
All rounds fired normally.

At this point, we recovered the spent brass and I took them home and resized the brass and caught the primers. I looked at them under a magnifying glass to see if I could see any "invasion" of oils into the primer pocket, but there was no evidence of any problem with any of them.

Week Four:
All rounds fired just fine.

Week Five:
All rounds fired just fine.

I checked the rounds that were left after the 5th week, and they all still had a puddle of oil on them, except for the rounds with WD-40 on them.

It was still there, but some of it had evaporated, as expected.

So, I re-applied it to those rounds.

Here's a picture to show the puddles.

Here's a picture to show the puddles.​

Week Six:
Today, we fired the remaining rounds (5 each) of each type of oil.

I decided to do a little practice on double taps, just to avoid wasting the ammo.

All 30 rounds fired just fine.


Lessons learned:

  1. 1. Was this test "fair"? No, absolutely not.
    It was about ten times more severe than any normal situation, where oil might come into casual contact with a cartridge. No one puddles oil on the primers of their ammunition for 6 weeks at a time. This test was a Worse Case Scenario.
    Might different ammunition and different oils react differently than those in this test? Yes, that is possible.
  2. Will casual oil likely invade and "kill" primers in cartridges? No. This rumor is greatly exaggerated.
  3. Is it possible that in some case, under special circumstances, that oil might kill a primer in loaded ammo? Maybe. But it would have to be a worse scenario than this one.
  4. Should we then just get oil all over our ammunition? No, of course not. There is no need to court disaster. I will continue to keep my ammunition as dry as possible.
But I won't be paranoid about it any more.

And shooting stuff is fun.

And there you have it, boys. Another myth busted.