Yes, it "does something."
Decoppering agents are not new. They were first used over a century ago, primarily in artillery, because of accuracy problems. After plenty of shooting through hot barrels, copper strips actually started hanging out of the muzzle.
After experimentation, it was discovered that adding lead to the powder charge vastly reduced copper fouling. The reason was eventually traced to the lead combining with the copper during the heat of firing, resulting in a brittle amalgam that blew out of the barrel.
Back then the lead was often added as a coil of wire on top of the powder charge before the projectile was seated, the amount of lead being adjusted to the specific load. Eventually it was discovered that other metals had the same results, especially tin and bismuth.
Quite a few early rifle powders had tin added, such as the "1/2" series of early DuPont powders. But evidently the amount of tin was often too much, which resulted in tin fouling instead of copper fouling--and the tin was harder to remove.
Decoppering agents have often been added to the Ball powder developed by Olin in the 1930's, primarily because they were often used in ammo for automatic rifles, where copper built up quickly. ("Ball powder" is a trademarked name; the generic name is spherical.) In fact they've been incorporated in some Ball powders for a while, though nobody made a big deal of it.
From what I can extract from powder manufacturers (who are naturally shy about revealing secrets), the probable decoppering agent in all the recent powders is bismuth, which unlike lead is non-toxic. They very definitely work, though how well depends on various factors, including how much powder is used compared to the bore-size, bearing surface and copper-composition of the bullet, velocity, bore diameter and smoothness, etc.
However, they always reduce copper fouling, and sometimes eliminate it. I recently got done with a project involving new powders and bullets in the 7mm Remington Magnum. I'd shot the test-rifle enough beforehand to know its barrel fouled some, due to inspecting it with my Gradient Lens Hawkeye bore-scope. But the majority of the powders tested included decoppering agents. After well over 100 rounds without cleaning, there was ZERO copper-fouling in the barrel.
That doesn't always happen, due to the powder/bore/bullet factor, but yes, decoppering agents do work, and have for over a century.
