I drill through UL glass everyday and don't have problems. The only time I've seen issues is in areas of higher humidity where your core material or wedges pick up moisture through the limb bolt hole after the limbs are made and expand, causing the glass to split. To avoid this, seal the inside of your limb bolt hole with a Q-tip and finish or some cyno. Like James V said, back your limb up with a scrap piece of wood and drill with a sharp bit. I use a regular drill bit.