The indoor range where I shoot (and have done RSO work) is generally OK with steel/aluminum
cases, but steel
core ammo is a no-no, and yes, there is a magnet to check. The magnet is generally for demonstrating to the person who brought in a bunch of Tula that yes indeed, those are steel core. Anybody who sees a steel round hit the backstop (especially something like .223/5.56) knows it's steel without the magnet.
Steel core rounds make more sparks (compared to lead core stuff). Steel .223/5.56 and 7.62X39 make a
LOT of sparks. Any unburned powder residue is obviously an issue. A busy range that's open for 10 hours can have a lot of powder on the floor toward the end of the day. True, any sparks with powder residue on the floor are risky (which is why this range sweeps the floor daily, and HEPA vacuums it twice per week). Steel just makes things a little more risky.
Handloads can be an issue because one never knows how well those were done save for the person who loaded them. True, the vast majority of people do this well (and safely) - as always it's the boneheads that create the issues.
The
linked video is not from this range - I just happened to find it online. Note how fast the fire propagates.