What makes you think they "hollowed out" the gold bars in the first place. Far simpler just to make a cast of the gold bar, make a mold, melt the gold, and pour it in over the tungsten bars. No need to "bore" anything.
All of the articles I have seen on these tungsten filled fakes say that they used a real gold bar and hollowed it out, or drilled it out.
They use a real gold bar from a reputable house because it ensures that they are getting real gold in their scam and it makes their fake almost undetectable.
The fake gold bar has a real seal and a real serial number and is correctly marked. They don’t have to try to duplicate any of that.
The hollowing out of real gold bars is what is really worrying folks. A cast fake would be much easier to spot. Duplicate serial numbers would be noticed.
“What makes it so devious is a real gold bar is purchased with the serial numbers and papers, then it is hollowed out, the gold is sold, the tungsten is put in, then the bar is closed up. That is a sophisticated operation.”
http://www.theblaze.com/stories/fake-gold-bars-discovered-at-store-in-midtown-manhattan/