Like any disposition, a licensee has 7 days (478.125(e)) to enter the disposition of destroyed items.
ATF has provided clear instructions that the preferred method of destruction of a receiver or frame should be by cutting with a torch or blade in 3 places, melting in a furnace, or smashing (if it truly compacts the receiver or frame).
https://www.atf.gov/firearms/how-properly-destroy-firearms
When a firearm is destroyed, there is no provision in the statute, regulations, or rulings that requires a photograph to be taken of the destroyed firearm or for there to be witnesses. However, FFLGuard recommends taking pictures, having witnesses, and saving them.