Heat guns can be tricky on old shellac for sure. Maybe try denatured alcohol with some fine steel wool first, it dissolves the shellac without the heat risk.
Oh man, heat guns and old furniture are a dangerous combo honestly. I tried that same trick on a dining chair from the 40s and ended up with a permanent burn mark that looked like a weird tattoo. @adam_thomas probably knows the pain of watching your project go up in smoke literally. What worked for me was using denatured alcohol and letting it sit for a few minutes before scrubbing with a plastic scraper. The shellac just melts right off without any heat damage, plus it smells way less like a campfire. Just make sure you've got good ventilation cause that stuff is potent.