Test Subject: 55gr .223 Remington FMJ
Target Medium: CRS High Density Vulcanized Ballistic Rubber
Introduction & Executive Summary
When a high-velocity rifle round impacts dense rubber, it experiences extreme fluid-dynamic drag forces. In a recent empirical live-fire test, twelve 12″ × 12″ × 1″ ballistic rubber panels were stacked to create a 12-inch target block. A standard 55-grain .223 Full Metal Jacket (FMJ) bullet fired at a nominal muzzle velocity of 3,200 ft/s penetrated exactly 6 inches deep before coming to a complete stop.
This article details the quadratic drag model governing bullet deceleration, explores the physical differences between stacked individual panels and solid rubber blocks, and predicts exit speeds across 1.0″, 1.5″, and 2.0″ solid rubber barriers.
1. Velocity & Energy Dissipation Across the Stacked Panels
A common misconception in terminal ballistics is that a bullet loses equal speed per inch of material penetrated. In high-density rubber, resistance is governed by quadratic drag, meaning drag force is highest when velocity is highest.
As a result, kinetic energy is dumped almost linearly over distance, which causes velocity loss to accelerate dramatically near the end of the penetration path.
Panel-by-Panel Breakdown (1″ Stacked Panels)
| Panel # | Depth Traversed | Entry Velocity | Velocity Dropped | Retained Energy | Kinetic Energy Lost |
|---|---|---|---|---|---|
| Panel 1 | 0″ to 1″ | 3,200 ft/s | 291 ft/s (9.1%) | 1,032 ft-lbs (82.5%) | 219 ft-lbs |
| Panel 2 | 1″ to 2″ | 2,909 ft/s | 299 ft/s (9.3%) | 831 ft-lbs (66.4%) | 201 ft-lbs |
| Panel 3 | 2″ to 3″ | 2,610 ft/s | 348 ft/s (10.9%) | 631 ft-lbs (50.4%) | 200 ft-lbs |
| Panel 4 | 3″ to 4″ | 2,262 ft/s | 414 ft/s (12.9%) | 430 ft-lbs (34.4%) | 201 ft-lbs |
| Panel 5 | 4″ to 5″ | 1,848 ft/s | 541 ft/s (16.9%) | 215 ft-lbs (17.2%) | 215 ft-lbs |
| Panel 6 | 5″ to 6″ | 1,307 ft/s | 1,307 ft/s (40.9%) | 0 ft-lbs (0.0%) | 215 ft-lbs |
In the first panel, the bullet loses under 300 ft/s. In the sixth panel, it loses its remaining 1,307 ft/s to come to a dead stop.
2. Solid Block vs. Layered Panels: The Gap Effect
A critical finding from this research is that a continuous, solid 12-inch block of rubber will stop a round roughly 20% faster than twelve stacked 1-inch panels.
While stacked 1″ panels allowed the bullet to travel 6.0 inches, a single continuous block of the same density stops the bullet at ~4.8 inches.
[ STACKED 1" PANELS ] ──> Air gaps vent shockwaves ──> Deeper Penetration (6.0")
[ SOLID RUBBER BLOCK ] ──> Trapped cavitation pressure ──> Faster Stop (4.8")
Why Does the Solid Block Stop the Bullet Faster?
- Hydrostatic Venting: As the bullet creates a temporary cavity, high-pressure rubber walls push back violently. In stacked panels, air gaps between layers allow this pressure to vent laterally, reducing continuous hydrodynamic drag.
- Structural Flexing: Thin 1-inch panels flex and bow forward slightly before tearing, converting bullet momentum into elasticity. A rigid, continuous mass cannot flex easily and forces immediate shearing.
- FMJ Yaw Stabilization: High-velocity FMJ bullets rely on tumbling (yawing) sideways to dump energy. Air gaps temporarily relieve lateral forces, allowing the FMJ bullet to stay pointed straight longer before tumbling.
3. Predicted Speeds Through Solid Rubber Panels
Adjusting for a solid high density rubber block, we can calculate the exact exit velocities for common standalone panel thicknesses:
Exit Velocity Comparison
- 1.0-Inch Solid Rubber Panel:
- Entry: 3,200 ft/s | Exit: 2,848 ft/s
- Drop: 352 ft/s (11% reduction)
- Energy Retained: 79.2% (991 ft-lbs remaining)
- 1.5-Inch Solid Rubber Panel:
- Entry: 3,200 ft/s | Exit: 2,654 ft/s
- Drop: 546 ft/s (17% reduction)
- Energy Retained: 68.7% (859 ft-lbs remaining)
- 2.0-Inch Solid Rubber Panel:
- Entry: 3,200 ft/s | Exit: 2,443 ft/s
- Drop: 757 ft/s (24% reduction)
- Energy Retained: 58.3% (729 ft-lbs remaining)
4. Key Takeaways
- Quadratic drag dominates: Velocity decay is non-linear. The final inch of material does significantly more deceleration work (in terms of ft/s dropped) than the first inch.
- Solid beats stacked: If designing backstops or armor shielding, monolithic solid blocks are ~20% more efficient at stopping rifle rounds than stacked sheets of identical combined thickness.
- Thin rubber won’t stop rifles: A 2-inch solid rubber block strips 757 ft/s off a .223 FMJ bullet, but the exiting round is still traveling at a lethal 2,443 ft/s.
(The attached PDF includes full publication-grade graphs comparing the velocity decay curves, kinetic energy loss profiles, and solid panel exit speeds.)
