How can I get paint to come off more evenly with a heat gun?
Perhaps you should consider using a chemical help to supplement your heat gun approach. Obviously the very old paint has lost most of the oils and resins that can help it bubble for heat removal. You are probably dealing with mostly pigments (many metal based, including lead) and very dry resins.

There are newer less caustic strippers, such as citrus based products, that take several hours to cut through the old paint and may need a subsequent application. But they may allow more complete removal without gouging the old wood. 

 

Once you stop making satisfactory progress with the heat gun, let the wood cool than then test the stripper in a small area. NOTE - Do not, under any circumstances, use the heat gun when there is a chemical stripper on the wood. Serious chemical reaction or fire could result!

You also could try cabinet scrapers. these are much less likely to gouge than the heavy duty paint scrapers. Some have sharp corners or curves that allow you to scrape out moldings and insets. Again, these could be used once you have removed a good bit of the old paint with the heat gun.



NOTE 2 - Given the age of the paint, there is almost certainly lead (and maybe other unpleasant metals) present in the finish. Use a mask to avoid breathing the dust and chips with all these techniques.