1/8" Carbon Steel NPT Tap and "R" High Speed Steel Drill Bit - Walmart.com
Drill Size R, Dec Equiv 0.339 | Abbeon
Drill Bit Size "R" M42 Cobalt Heavy Duty Premium Quality 1-pk
6.1 Guhring 1242 Solid Carbide Drill Bit DIN 6539 R-RT1 11089 NEW | eBay
In case anyone doesn't know this(I certainly didn't), drill bits for glass work pretty well drilling through hardened steel. More info in comments. : r /Blacksmith