Here are two AR15 chamber flags I designed in TinkerCad. These are not meant to take the full weight of the bolt slamming against them. I like to lock my bolt back and slip one of these in thru the ejection port and down thru the magwell. It makes it easier for Range Officers to see that the bolt is open and clear. I recommend using PLA+ or something stronger than regular PLA. These print in about 22 minutes each at 100 percent infill on my Ender 3 V1 using Chep's extra fast Cura profile. Search YouTube or Google to find it.

Creality

Ender 3

No

No

100