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The market for 7.62x51 ammo is so depleted of supply you’re starting to see third-world 7.2x51 ammo that is corrosive come into the markets. Usually it will have a potassium chlorate primer compound, and unless you clean your barrel promptly and thoroughly after using it, your barrel will start to corrode and pit.

OK, some history so you can put this into context:

100 years ago, most all smokeless ammo in the US was corrosive - they used a mercury-based primer compound, mercury fuliminate.

The mercury-based priming compounds would erode your barrel, not corrode them - but the brass was a throw-away after using a mercury fulminate primer. Very often, once-fired brass using a mercury primer would fail on the second or third loading. I won’t get into the metallurgy here, just suffice to say that mercury combines with metals like copper, tin, brass, etc under pressure and the fundamental nature of the brass changes as a result - it becomes brittle. The point being that mercury priming caps had been used for a long time with black powder guns and it worked OK - it was stable, kept in storage, worked pretty well. It was just the higher pressures of smokeless that caused problems with mercury fulminate. You could shoot mercuric primers and not lose your barrel.

Back then, the military was re-loading their practice brass on US training ranges. This business of the brass failing (resulting in injury and weapon damage) was very much annoying, so the Ordnance Dep’t found the problem and changed the priming compound: the military switched from mercury to potassium chlorate. By the post-WWI years, mercuric priming was history on most all US military small arms loading - but not on Communist Bloc stuff, because as we all know, the commies often used cheaper steel for their cases, which are not reloadable, and they don’t care if their primers compromise their cases.

Potassium chlorate IS corrosive to your barrel steel - potassium chlorate, under heat and pressure, would change into potassium chloride, which is a salt (it is actually sold as a salt substitute for people on low-sodium diets) and it highly corrosive to steel. Shoot a few rounds of potassium chlorate primed ammo, then allow your rifle to sit for a couple weeks. Your barrel will be wrecked. This is why our grandfathers were beat by the DI’s into cleaning and oiling their weapon every day.

By the late 1920’s, most civilian ammo was made non-corrosive by switching to lead styphnate. You could see in the old magazines of the day Winchester and Remington were advertising the fact that their ammo was non-corrosive — eg, the Remington trade name “Kleenbore” on their ammo - that was their non-corrosive primer coming into use.

The military was still using corrosive priming compounds into the early 50’s. The Commies were using corrosive priming compounds into the 1980’s.

Now the modern, non-corrosive priming compound lead styphnate has become cause for concern in indoor firing ranges, and there is another compound used in primers marked “lead free” but you probably won’t see those compounds on mil-surp ammo.

Moral of the story: if you get surplus ammo from some backwater country, even of recent (1980’s) production, CLEAN YOUR RIFLE AFTER SHOOTING THE AMMO.