Some careful planning is needed prior to drilling your holes. At the lug location, measure the barrel's diameter, and subtract the bore including the groove depth X 2. Now divide that dimension by 2 and you have how much steel there is from the surface to the bore. Eg: swamped barrel in .50 cal. with .012" deep rifling. Diameter at the staple is .870". Subtract the bore of .50 cal. = .370". Groove depth is .012" x 2 = .024". Subtract this from .370" = .346". This includes metal both above and below the bore, so divide by 2 = .173". At this location, you have .173" of steel from the surface to the bore. The leg on your lug is likely only about .097" or 3/32". Drill your holes .100" deep and you still have .073" of steel between the end of the hole and the bore.
How much is too much? The pounds per square inch of bore pressure on the end of that hole is practically insignificant. Still, you don't want the bore to dimple from outward pressure, or from hammering the barrel metal into the hole either. So I'd stop drilling at ~.060". If your barrel is too skinny for this type of fastener, use a dovetail or solder on a lug.
Finally, I use a nail set ground flat on the bottom and with a flat side (so I can place the set right up against the staple) and drive in three places...end and each side forming a cloverleaf. The tool TOW sells is theoretically great, but requires too heavy a hammer blow to move enough steel into the cavity, thereby jeopardizing the bore.
I edited my dimensions thanks to the eagle eye of a sharp patron. And that simply shows that he understands the concept and can do math better than I. It also shows that you have to double check your math before you drill!!