Frangible bullets are made by pressing powdered metal into bullets under intense pressure.
They are usually made out of powdered copper or even sometimes powdered steel with a plastic binder.
When they hit a hard target they turn to dust again.
Generally speaking, they are used in indoor ranges to prevent lead dust.
They are used on outdoor rifle ranges to prevent damaging the steel targets. Or to prevent close range bounce backs hitting the shooters
Might also be a ricochet problem at your rifle range they are trying to get control off?
If you are a member, they surely sent out letters or something to explain why they had to adopt a rule like that!!
Or you might ask one of the officers what the heck the reason is?
rc