Arrange peppers on a baking sheet coated with cooking spray. Broil 6 minutes or until blackened in spots, turning occasionally.
Remove lid from a 6-quart Instant Pot. Press [Sauté]; use [Adjust] to select “More” mode. When the word “Hot” appears, coat inner pot with cooking spray. Add beef; cook 5 minutes, turning to 1⁄4 teaspoon grated whole nutmeg
Add peppers to pot; stir in, vinegar and next 3 ingredients. Stir in 1⁄4 teaspoon of the salt, black pepper, and chopped pepperoncini. Return beef to pot.
Close and lock the lid of the Instant Pot. Turn the steam release handle to “Venting” position. Press [Slow Cook], and use [Adjust] to select “Less” mode. Press [-] or [+] to choose 8 hours cook time.
When time is up, remove beef from cooker; shred with 2 forks. Stir beef and remaining 1⁄4 teaspoon salt into cooking liquid. Divide beef mixture evenly among hamburger buns, using a slotted spoon. Serve any extra sauce for dipping.