We have shot 10s of thousands of frangible rounds over the year during quals. Most of it BC223NT but when DoD guys go through our SWAT school many times they will bring Mk255 or at least what looks like it as I recognize the grey tip rounds. Frangible is a hard round. It marks up my steel worse than the 62 grain Q3315 we use. Frangible may break up on steel but it goes right through the wood and other stuff. I would never think it was a safe round indoors. There is no AR500 steel in my house.
I remember a few years ago I was using another Agency's range. We were using Federal BC223NT. One of the last things out of my mouth to the shooters was try and not hit the carriers, we are using someone elses range and we don't want to shoot it up. I swear I was running one on one shooters through a shot and move course and the VERY FIRST ROUND hit the tube steel target carrier and almost cut it right in half. I couldn't believe it.
SWAT has also shot at some handgun steel plates mistakenly thinking they were rifle plates. Frangible ammo makes deep craters in it.
Also we rarely use frangible pistol ammo on our indoor pistol range. The taller shooters shooting a the 3 yard line have their round striking the floor at about 20 yards down range. The frangible pistol ammo always take chunks out of the concrete range floor. Also try shooting some BC223NT at cinder blocks. It blows them to pieces better than our Q3315 ammo.
Frangible ammo is quite hard amd penetrates deep until hitting something solid like hardened steel. David