Experiments with oil and primers

A week or two ago Ry showed up in my office and said he wanted to do an experiment. He and I have had conflicting information on the contamination of primers by oil. The common word on the Internet and word of mouth is that if you get the tiniest amount of oil on the chemically active portion of a primer then it would go dead.

However Ry had seen Lyle demonstrate this was not true by putting CLP in a shell casing with an active primer then detonating it a minute or two later. I had done similar things myself. I wanted to deprime some cases with live primers and to test the “kill the primer with oil” hypothesis I put a thin oil in the mouth of a shell casing and waited, sometimes a day or more, but they would still fire just fine. Bah! As usual the Internet is wrong. Right?

As we talked about the experiment we realized the Internet story wasn’t quite the same thing as our first hand data. Our first hand data wasn’t of putting oil directly on the primer. It was putting oil in the mouth of a shell casing. We believed the thin oil would make it through the flash hole into the active area of the primer.


But that was an assumption. It was not a proven fact.

So what we decided to do was soak the primers for few minutes in a paper cup and then load them in a shell casing. We would do several primers then test them at various time intervals, like an hour, a day, a week, and a month. We expected the primers would still be fine after a month and the myth would be busted.

Here are the solvents we tested:


We used Winchester small pistol primers:


Here are the primers being soaked prior to inserting on the empty shell casings:


An interesting thing occurred. The primers soaked in Break Free CLP turned the CLP slightly red after a few minutes:


And the primers in the water turned the water slightly yellow:


We also created two control sets. One set had dry primers in a dry shell casing and the other set had dry primers with solvent put in the shell casing.

After we had all the shell casing loaded Ry added more of the solvent to the test shell casings so evaporation would not be a factor in the long term testing.

We then tested both control sets and the test set.

We hand loaded the empty shell casings into a handgun and put the muzzle tight against an air pillow used for padding in Amazon shipments. We then fired the gun. If it failed to fire we would cock the hammer and try a second time. If it failed both times we called it a dead primer. There were no primers which failed on the first attempt and succeeded on the second. If it popped the air pillow it was a “vigorous” detonation. If it failed to puncture the air pillow it was a “mild” detonation:


The dry normally inserted primers would punch a hole through both sides of the air pillow even though there was a significant air gap between the two sides:


We tested two primers for each solvent. The results with solvent only in the shell casing mouth were as follows:

Solvent Detonation Result
Water 2 vigorous
Break Free CLP 2 vigorous
WD 40 2 mild
3-IN-ONE 1 vigorous, 1 mild
Tetra Gun Lubricant 2 vigorous

This confirmed the tests Lyle and I had done years ago. The primers were still active after putting oil in the case mouth.

Here are the results of the primers soaked in the paper cups with solvent:

Solvent Detonation Result
Water 2 dead
Break Free CLP 1 mild, 1 dead
WD 40 1 mild, 1 very mild
3-IN-ONE 2 mild
Tetra Gun Lubricant 2 mild

We will have updates later after the remaining primers have soaked for longer periods of time but it is clear that the flash hole is a significant barrier to the entry of solvents into the primer compound and the common wisdom of oil damaging primers is true.

Update from Ry after one day:

Solvent Detonation Result
Water 2 dead
Break Free CLP 1 mild, 1 dead
WD 40 1 very mild, 1 dead
3-IN-ONE 1 mild, 1 dead
Tetra Gun Lubricant 2 dead

16 thoughts on “Experiments with oil and primers

  1. Makes sense. Primer holes are small enough that surface tension of the contaminant doesn’t allow it to break and leak into the primer under normal circumstances. Vibration might do that though, especially over time.

    • Yup. There is an air pocket without an exit. How is the air going to get out at the same time the oil is going in?

  2. Several years ago, someone at Arfcom did a similar test with 5.56 ammo and water. Everything from military M193 by Federal to steel cased Wolf and Tula, crimped/uncrimped primers, sealed/unsealed bullets/primers, soaked in water long term vs heavy humidity etc. Unsurprisingly if memory serves, the military rounds with sealant did pretty well. The steel cased stuff was awful, didn’t handle water well at all.

  3. I’ve done similar tests in the past, concerning primers “handled with oily fingers” during the reloading process.

    My tests were not as ‘scientifically rigorous” as yours. However my (nearly anecdotal) tests did not … repeat NOT! … show that ‘oily fingers’ were in and of themselves detrimental to the ability of the primers to ignite powder charges in .45ACP cartridges.

    My assumption is that would be even less detrimental in cartridges using ‘small’ primers.


  4. Perhaps this is due to changing primer chemistry. The Wikipedia article on “centerfire ammunition” has a fascinating section on primer chemistry. Suffice it to say that primer chemistry has changed significantly several times in the preceding half century.

  5. Yes, the underlying chemistry would be very interesting. Consider the old classic, mercury fulminate. That’ insoluble in water, so one would expect it to be less affected. Some other substances (like perchlorates) are very soluble in water.
    Mercury fulminate is also insoluble in ethanol and ether, which makes me wonder how oil would affect it. A possible answer is that it affects the grain structure, or the friction or heat between the grains.
    What makes this curious is that oil (diesel fuel) is a standard ingredient of some explosives.

  6. Fully assembled cartridges SHOULD be largely impervious if they’re done right. There have been millions and millions of Russian 7.62 x 39 hollow point rounds sold with a lacquered case mouth, lacquered primer, and the bullet core so loose in the jacket that you could blow through it like a straw. Ask me how I know. Most HP has a closed base, but for some reason this Russian HP was open at both ends, and the core would rattle inside the jacket.

    Just as important would be the powder’s response to solvent incursion, or ultimately, the techniques for assembling impervious ammunition.

    I was told by an instructor once that you should totally avoid the use of penetrating oils when cleaning or lubing your gun because it could end up penetrating (and deactivating) your carry ammo. That might have been what prompted all this.

    • The other reason to avoid penetrating oil (i.e., WD-40) is well known to machinists: contrary to its marketing, WD-40 is NOT a lubricant. The best one-word description may be “glue”. 🙂 It’s a mixture of substances, some of which evaporate quickly, leaving a thick sticky residue. If you have a mechanism and you squirt WD-40 in it, it will work nicely for a short while, then it will stop working and require a lot of trouble to get it unstuck.
      WD-40 has no place around mechanisms. Its only real use is the last step before a torch in removing rusted fasteners.

      • Maybe not its *only* use: it works quite nicely as a solvent to clean “way” oil on machine tools. Also seems to work as a cutter coolant/lubricant when cutting aluminum. But to your main point: I wouldn’t use it anywhere I needed “long term” lubrication where “long term” means more than a couple of seconds.

  7. Interesting. Here is a side experiment I ran unintentionally. I bought some brass years back for an oddball caliber, marketed as ready to load. I normally would wash and clean the brass but I was running late, and it was supposedly ready to load (famous last words on that one). The short story is that I ended up with 10 rounds that had been thru my normal process and ~10 rounds that went straight from bag to loaded round.

    Every single round I loaded from the cleaned brass fired properly. Every single round from the “straight from the bag” pile failed to fire. Primer detonated, drove bullet into lands, left mess of powder in rifle when extracted. Something, some residue on the cases prevented the ignition of the powder.

  8. So the firearm likes oil, which can degrade the ammo. With black powder it’s even more of an issue, since the large amount of corrosive fouling means you need more grease and whatnot in the mix. Keeping your powder dry and keeping the barrel steel protected are not conflicting goals though, if you use the right stuff. Last night I was assembling cartridges with grease placed inside the case, right against the powder, and I have full confidence in their reliability so long as they’re kept below the melting temperature of the mutton tallow until firing.

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  10. Pingback: Experiments with oil and primers, part the second | Bacon, Alcohol, Tobacco, Firearms, Explosives

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  12. Pingback: Experiments with oil and primers, part three | Bacon, Alcohol, Tobacco, Firearms, Explosives

  13. Is there any way to restore primer function after they have been “killed”?

    Will wet primers, once dried out, pop? can oily primers ever be restored to any sort of function?

    Maybe some chemists can shed some light on further experiments.

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