15N20 is very horrible for liners.... simply because as it comes, it's "shereodized annealed"....... in other words, half way annealed.
If you have the means, place it in a heat treat oven at 625-650F and soak for 2 hours, then allow it to cool down NATURALLY with/in the oven.
If you use a kitchen type oven do the same, but after two hours, just shut it off, DO NOT OPEN THE DOOR, and go to bed. In the morning is MIGHT be soft enough to drill.
Even with those methods, 15N20 is often hit-n-miss.... it's just a difficult steel to fully anneal.
In the future, I would suggest using another material for liners.... like 410SS or Titanium. Don't be afraid of Titanium..... there is a small learning curve with using it, but once you understand it's working characteristics..... they are absolutely the same each and every time, with each and every piece of Ti..... of course assume its the same alloy (6AL4V is the most common Ti alloy used by knifemakers). That's unlike the 15N20, which always has you guessing, because not two piece are the same.
