This is a .22LR snap cap to protect your firing pin from excessive wear or indeed breakage if using a pistol chambered in .22LR. My Walther PPQ M2 22 4" will readily eject this so I recommend printing in bright colours such as red or orange so you can retrieve if you need to. Probably best printing this using a high impact filament such as Nylon (Polyamide) or PETG as it's crucial to choose a high-impact, durable filament for these.