Corrosive/Non-Corrosive Ammunition

Corrosive ammunition uses a primer that contains potassium chlorate. When ignited, it leaves a residue of corrosive salts in the bore and chamber after firing. Most modern ammunition is not corrosive (dated military surplus ammo is sometimes the exception). 

Most militaries were aware of the corrosive downside of potassium chlorate before World War II. They distrusted or could not afford the new (and more expensive) non-corrosive primer formulas and continued loading their cartridges with corrosive primers during and after the war. Today, it is nearly impossible to encounter newly manufactured corrosive ammunition, but billions of corrosively primed cartridges are still in circulation as military surplus.