[Sturmgewehre]

The bi-metal bullet can be cause for concern either outdoors or indoors.

Indoors you have unburnt powder residue sitting on the floor. That blackish ash on the floor of your local range isn't dirt, it's powder flakes. Most firearms spew unburnt powder out of the muzzle when they fire leaving small traces of powder on the floor. After thousands of rounds this adds up and can create a fire hazard. Bi-metal bullets have a lead core that's wrapped in mild steel. The steel jacket is then wrapped in copper. When the bullet strikes the steel traps sparks are created at times. These sparks can ignite the unburnt powder on the floor. That's why many indoor ranges ban the use of bi-metal bullets in their ranges.

Outdoors you can have a similar problem expect this time it's not unburnt powder that's the potential problem, it's dry conditions and an abundance of flammable tender. Bullets striking rocks can create sparks and set something on fire, it has happened.

The notion bi-metal bullets harm your barrel is unfounded. The jacket is mild steel, it's very soft. I've fired literally thousands of upon thousands of rounds through my rifles and handguns and seen no signs of excessive wear.