If you really want DIY bullet resistance, I think the earth bermed recycled tires filled with rammed earth takes the cake.
Add reinforcing mesh and a layer of gunnite for the exterior, and you've got a pretty cheap bunker as long as you don't count your sweat equity in your cost calculations. Since tires and earth are used as a backstop at gun ranges as-is, whatever gets through the gunnite will be partially deformed and not be doing well against the rubber, steel belts, kevlar thread, and compacted dirt after that.
IMO, even non-AP .50 BMG would probably be caught by the inside wall of the tires. A lucky hit between tires might get through, but IIRC, you zig-zag the tires two deep to make your walls and use a 1/2 alternating offset like bricks so it would have to be a perfect shot at a 45 degree angle, so the thickness of the wall is higher from the bullet's perspective, and the concrete would be thicker in the triangular joints between tires.
The one vulnerability/downside is that the "earth ship" type construction does not work on the principle of insulation, but on thermal mass, since the walls are quite thick. So you generally want some kind of south facing exposure, a solarium or greenhouse to collect solar heat in winter.
