What is the difference between corrosive and non-corrosive ammo?
The difference is the primer, not the bullet, powder or case. SAAMI, the industry standards body, defines a corrosive primer as "a priming mixture that contained compounds of chlorine and oxygen generally used in military ammunition made before 1952. The residues are hygroscopic and, therefore, promote rusting." A non-corrosive primer is "a primer which does not contain chemical compounds that could produce corrosion or rust in gun barrels" (SAAMI glossary).
Firing either one leaves fouling in the bore and action. Fouling from a non-corrosive primer is ordinary dirt. Residue from a corrosive primer contains a salt that draws moisture out of the air, and that moisture rusts steel. Corrosive ammunition is therefore safe to fire in a firearm chambered for it; the consequence is what happens to the firearm afterward if the residue is left in place.
What makes a primer corrosive?
The compound responsible is an oxidizer. In the description given by Frank Iannamico in Small Arms Review, "the suspect component is potassium chlorate (KClO3), which is used as an oxidizer, providing oxygen to the primer compound. When ignition takes place the oxygen is removed from the molecule leaving potassium chloride (KCl). Potassium chloride is a salt and therefore hygroscopic (attracts moisture)" (Small Arms Review, "Removing the Mystery from Corrosive Ammunition").
That chain explains the behavior people associate with corrosive ammunition:
- The primer fires and the chlorate gives up its oxygen.
- Potassium chloride, a salt, is left as a fine deposit in the bore, chamber, bolt face and anywhere else the gas reaches.
- The salt absorbs water vapor from the air, and the wet salt corrodes the steel underneath it.
In the same article's test, steel exposed to corrosive primer residue "began to rust after 24 hours," and by 72 hours the metal had begun to pit. The article summarizes that, left untreated, corrosive primer residue results in rust "within 24 to 48 hours." Humidity changes how fast this happens; the chemistry does not change.
How do corrosive and non-corrosive primers compare?
| Attribute | Corrosive primer | Non-corrosive primer |
|---|---|---|
| Defining compound | Chlorine-and-oxygen compounds such as potassium chlorate | No compounds that produce corrosion or rust in the barrel |
| Residue after firing | Hygroscopic salt (potassium chloride) | Ordinary fouling |
| Effect of leaving residue | Rust on exposed steel, then pitting | No corrosion attributable to the primer |
| Where it is found | Older military production, foreign surplus, some blanks | U.S. military production after the 1950s transition; ammunition its maker labels non-corrosive |
| Dissolved by oil-based solvents | No — salts are water-soluble | Not applicable |
| SAAMI reference era | "Military ammunition made before 1952" | — |
Sources: SAAMI glossary, Small Arms Review.
Where does corrosive ammunition come from?
Military production is the main source. SAAMI's definition dates the typical corrosive primer to "military ammunition made before 1952," and Small Arms Review reports that "most U.S. military cartridge contractors made the transition to non-corrosive primers during the 1950 era." Other countries' arsenals made the change on their own schedules, which is why the same article says corrosive primers remain common in "foreign ammunition, particularly surplus military cartridges and blanks." The ammunition it tested was surplus Romanian 7.62x25mm.
So the question is usually not about current commercial ammunition. It comes up with military surplus — ammunition made for a government, stored, and later sold off — in cartridges with long military histories, such as 7.62x39, .30-06 Springfield, 7.62x54R, 8mm Mauser and 7.62x25 Tokarev. Whether a particular lot is corrosive depends on who made it and when, not on the cartridge name.
How do you tell if ammo is corrosive?
There is no reliable visual test on a loaded round. The information that settles it is the maker and the production date, which come from the packaging, the lot documentation, and the headstamp — the markings stamped on the case head.
The Small Arms Review article's practical guidance is about listings rather than cartridges: "if it doesn't state that the ammunition is non-corrosive it probably is," and it warns readers to be wary of all foreign surplus ammunition "even if the seller or the box states that is non-corrosive." The article's caution is directed at foreign surplus.
Primer type is not the test. Berdan and Boxer are two primer designs. SAAMI defines a Berdan primer as one whose anvil "is an integral part of the cartridge case head," and a Boxer primer as a self-contained assembly of "a cup, explosive mixture, anvil and covering foil or disc" (SAAMI glossary). The design describes where the anvil sits and whether the case can be easily reprimed. It does not describe the priming mixture, which is what makes a primer corrosive or not.
Mercuric primers are a separate category. SAAMI defines a non-mercuric primer as "a primer which does not contain compounds of mercury" (SAAMI glossary). Mercury compounds and chlorate compounds are different ingredients; a primer can be described on either axis.
What does corrosive ammo mean for cleaning?
Because the residue is a salt, the cleaning question is how to dissolve and remove salt. Small Arms Review notes that "potassium chloride is soluble in water; making water an ideal liquid to remove the corrosive salts." In its tests, boiling water "worked very well and evaporated quickly," while water left to sit produced light surface rust of its own after several days. Petroleum-based products such as conventional bore solvents and penetrating oils did not dissolve the salts; they protected the steel only as a coating.
The parts that need attention are everything the gas touches, not just the bore. In the article's words: "Any parts exposed to the cartridge gases should be cleaned, particularly on blowback, open-bolt guns; the bolt face, inside of the receiver and feed lips of the magazines should all be thoroughly cleaned of powder residue." On gas-operated rifles the gas system is in the gas path as well. The corrosion test above sets the window: residue left in place had begun to rust within a day.
The sequence the article's results point to is: dissolve the salt with water or a water-based cleaner, dry the metal completely, then apply oil as a protective coating. The order matters because oil on top of undissolved salt does not remove the salt.
Does corrosive ammo cost less?
Not necessarily. Whether a given surplus listing is lower on a cost-per-round basis than current commercial ammunition of the same cartridge depends on the listings on that day. IronScout shows cost per round across tracked retailers for 7.62x39 and .30-06, and the 7.62x39 price history shows the observed range. Cleaning time and materials are a cost the per-round price does not include.
Primer chemistry and case material are separate attributes; the trade-offs of the case itself are covered in steel case vs brass case. Corrosion of the case in storage — rust on steel or deposits on brass — is a storage condition rather than a primer property, and is covered in ammo storage and shelf life.
Related articles
Sources
- SAAMI glossary — C — corrosive primer, centerfire primer
- SAAMI glossary — N — non-corrosive primer, non-mercuric primer
- SAAMI glossary — B — Berdan and Boxer primers
- Frank Iannamico, "Removing the Mystery from Corrosive Ammunition," Small Arms Review V12N9 (June 2009) — primer chemistry, corrosion test, cleaning test