I wouldn't recommend a HF paint gun, let alone drilling out the tip.
If you're going to do much painting, buy a decent paint gun. I have a 3M PPS gun that works well. Comes with a selection of tip sizes. The supplies (disposable cups, etc) are a little pricey, but the ease of cleaning (takes just a couple ounces of cleaner) makes it worth it for the occasional painter. Did I mention disposable?
