What Stops a Bullet? Brick vs Concrete vs Cinder Block
If rounds start flying, what around you is actually cover — and what just looks like it? We put the three most common masonry materials — red brick, reinforced concrete, and cinder block — through a physics-based penetration model, against both a 9mm handgun and a .223 rifle.
The results are counterintuitive. The wall that looks most substantial — the cinder block — is the one a 9mm punches straight through. Meanwhile a thin course of solid brick stops the same round cold. And a rifle? It barely notices the difference between any of them.
Bottom line: Against a 9mm, reinforced concrete is the best stopper (≈35 mm penetration into a 120 mm wall), red brick holds up well (≈65 mm), and cinder block fails — the round exits the far side and keeps going. Against .223, all three are defeated. None of them is reliable cover from a rifle.
The Test
Each material was modeled as a single 120 mm (≈4.7 inch) wall — roughly one wythe of brick, a poured concrete panel, or a standard block. Two rounds were fired straight-on from 10 meters:
9mm Luger — 115 gr FMJ, 7.5 g, 363 m/s at the muzzle (479 J at impact). The most common defensive and range handgun round in the world.
.223 Remington — 55 gr FMJ, 3.6 g, 988 m/s (1,713 J at impact). The standard AR-15 / civilian rifle chambering.
The single variable that drives everything below is compressive strength (σ_c) — how hard the material resists being crushed. Cinder block is the weakest at 20 MPa, brick sits in the middle at 60 MPa, and reinforced concrete is the strongest at 120 MPa.
Results: Penetration by Material
| Material | σ_c | 9mm FMJ | 9mm Result | .223 FMJ | .223 Result |
|---|---|---|---|---|---|
| Cinder Block (CMU) | 20 MPa | 160 mm | THROUGH | 460 mm | THROUGH |
| Red Brick | 60 MPa | 65 mm | STOPPED | 175 mm | THROUGH |
| Reinforced Concrete | 120 MPa | 35 mm | STOPPED | 168 mm | THROUGH |
Penetration depth into a 120 mm wall. "THROUGH" means the bullet exited the far side with velocity to spare.
Why the Cinder Block Fails
This is the result that surprises people. A cinder block wall looks like the most serious barrier of the three — it's thick, heavy, and grey like concrete. But against a 9mm it's the worst performer: the round drives 160 mm deep and exits the back of a 120 mm block still traveling around 138 m/s, punching an exit crater nearly 100 mm across.
The reason is in the name — concrete masonry unit, not concrete. A cinder block is a thin shell of low-strength aggregate (σ_c ≈ 20 MPa) wrapped around hollow voids. Its bulk is air. Reinforced concrete packs six times the compressive strength into a solid section. So while the block looks more substantial, the bullet is really only fighting a thin, brittle skin before it reaches empty space.
Solid brick, despite being the thinnest-looking option, has three times the compressive strength of the block's shell — enough to arrest a 9mm inside the first 65 mm.
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Switch to .223 and the careful ranking above collapses. The rifle round defeats all three materials — and here's the second surprise: it barely distinguishes between them.
| Material | σ_c | 9mm depth | .223 depth |
|---|---|---|---|
| Red Brick | 60 MPa | 65 mm | 175 mm |
| Reinforced Concrete | 120 MPa | 35 mm | 168 mm |
Look at brick versus concrete. For the 9mm, doubling the material's strength (60 → 120 MPa) nearly halves penetration (65 → 35 mm). The material matters enormously. For the .223, that same doubling changes penetration by less than 5% (175 → 168 mm). The material almost stops mattering.
The physics behind it: penetration into brittle material has a static term (the material's crushing strength) and a dynamic term that scales with velocity squared. A 9mm at 363 m/s lives in the regime where the material's strength dominates the outcome. A .223 at 988 m/s carries so much velocity-driven energy that it overwhelms the material term — the wall is almost incidental. This is why the old advice about "getting behind something solid" quietly assumes the threat is a handgun.
Cover vs Concealment
In defensive terminology, cover stops incoming rounds; concealment only hides you. The same object can be either, depending on what's being fired at it:
Against a handgun (9mm): reinforced concrete and solid brick are genuine cover. A cinder block wall is concealment only — it hides you but a determined 9mm goes through it with energy left over.
Against a rifle (.223): none of these single walls is reliable cover. All three are defeated. Stopping a rifle round takes far more material — think multiple courses of brick, thick poured concrete, or purpose-built ballistic barriers, not a single block wall.
The practical takeaway is uncomfortable but worth knowing: the barriers most people would instinctively duck behind are calibrated, at best, against pistols. The moment a rifle is involved, "solid-looking" stops being a useful guide.
Methodology Notes
Penetration was computed with the Poncelet cavity-expansion model with confinement correction, the standard analytical approach for brittle materials (brick, concrete, masonry). Each material carries measured physical constants — density, compressive strength, and fracture toughness. The 9mm depth into brick (≈65 mm) and the general ordering of materials fall within published field-test ranges for masonry; exact figures depend on the specific brick composition, mortar, aggregate mix, and block geometry, which a single-material model does not capture.
All results assume a straight-on (0°) impact into a clean, single-material wall. Real walls include mortar joints, rebar, air gaps, and construction layers that shift the numbers in either direction. These figures describe the physics of the material, not a guarantee about any specific wall.