Lead hazzard.
The nice hammer forged, super hard Glock barrels are designed to shoot jacketed bullets only. The barrel actually swages or squeezes the bullet down to barrel size, so FMJ is the way to go.
Lead is not recommended because lead particles and strips will tear off the bullet and be deposited in the polygonal rifling.
The easiest way to shoot lead, lots of lead, is to switch out the truly nice factory barrel with a conventionally rifled barrel.
The very nice people at Lone Wolf Distributing make the best aftermarket barrels for all Glocks. Give them a try...