Removing the firing pins would make the gun at least as inoperative as removing the bolt of a bolt action firearm. But the Regs say "bolt", not "bolt or its equivalent". I'd suggest following mooncoon's suggestion. There are also trigger locks that can be secured to a wall. As far as that goes, take a standard Master type lock, and drill a hole through the non-keyed portion, and screw that to a wall stud. But a couple of locked hoops look more secure. Make sure the gun is inoperative and tamper resistant. Also, keep in mind that a displayed firearm can invite a burglary. Make sure it can't be seen from a window.