At the U.S. Army’s Aberdeen Proving Ground, a team of ballistics technicians, supported by some of the U.S. Army’s top sniper instructors, has been quietly developing a radical new “dimpled” bullet. The exterior of the bullet resembles the dimpled surface of a golf ball. The function of the special dimpled skin is to reduce projectile drag, providing a flatter long-range trajectory, and greater retained energy at the target.
.338 Projectile with MIM Exo-Jacket
(3D-CAD Artist’s Rendering) army bullet
In their pursuit of a lower-drag bullet, the Army tried a variety of designs, including bullets with circumferential drive bands, dual-radius ogives, and rebated boat-tails. The dimpled “golf-ball” design was considered a “long shot” according to the design team, but it has performed beyond all expectations. The nominal drag coefficient (Cd) has improved by about +.040, while cartridge muzzle velocity has increased by nearly 80+ fps because the bullet’s dimpled skin reduces in-barrel friction. What’s more — the terminal performance of the dimpled bullet has been “spectacular”. The Aberdeen team set out to produce a slightly more slippery bullet for U.S. Army snipers. What they ended up with is a bullet with dramatically enhanced long-range ballistics and superior killing power on “soft targets”.
army bulletLt. Col. Ben Eldrick, Long-Range Projectile Project (LRPP) team leader, told AccurateShooter.com how the radical bullet was conceived: “During our initial design work, we wanted the benefits of a high-BC, pointed bullet, but in a design that could be mass-produced and could work as a tracer. We consulted some of the top civilian bullet experts, including ballistician Bryan Litz of Berger Bullets. Mr. Litz really got the ball rolling. He suggested that the ‘next big step’ in bullet design would involve the turbulent boundary layer over the body of the bullet. Litz told us that ‘pointing bullet tips will take you only so far… think about optimizing the airflow over the entire bullet’. That made a lot of sense to us. When you design a race car to be aerodynamic, you sculpt the whole body, not just the front bumper.
”
Lt. Col. Eldrick continued: “It turns out Litz was right on the money. By employing a golf-ball type dimpled surface, we were able to optimize the turbulent boundary layer on the bullet body. This reduced the low-pressure wake zone behind the bullet significantly, resulting in reduced base drag. As a result the bullet experiences much less overall drag, effectively raising the BC.” The Army team had discovered that what works for golf balls also works for bullets.
army bulletAfter testing a series of prototypes, the Aberdeen bullet design team settled on a copper-jacketed bullet with dimples about 0.5 mm in diameter. The first-generation bullets were formed in special binary impact swages that press-form the dimples after the bullets were pointed up in conventional dies. Future production bullets will be made with an advanced metal-injection-molding (MIM) process that forms the dimples directly into the surface of the bullets. Rather than simply wrap the core material (which is classified), the MIM is molecularly bonded to the core. The Aberdeen LRPP team calls this “Exo-Jacket” construction, as in “Exo-Skeleton”.
Higher Velocities Achieved
There was a surprise benefit of the dimpled bullet design — higher muzzle velocities. Given the same powder charge, dimpled bullets exit the muzzle faster because they produce less in-barrel friction than do conventional bullets. This is because the recessed dimples effectively reduce the metal-on-metal bearing surface. Lt. Col. Eldrick revealed: “the added velocity was an unexpected bonus. With equal-pressure loads, dimpled .308 bullets will fly about 80 fps faster than normal .308 bullets. With the large .338-caliber projectiles, the difference is even greater… we can pick up nearly 150 fps.” Given the observed velocity gains achieved with dimpled bullets, Aberdeen designers are now working on dimpled shell casings for larger artillery projectiles.
Dimpled Jacket Delivers ‘Explosive’ Fragmentation
While the internal design and core materials of the new dimpled bullet remain classified, the design team revealed that the terminal performance of the new bullet has been “spectacular”. The bullet penetrates like a FMJ but then explosively fragments, resulting in a devastating energy release in the target. According to Capt. Jack Sarazen, an Aberdeen engineer, “the enhanced terminal performance of the dimpled bullet was unanticipated. This was a serendipitous effect of the slight thinning of the jacket material where the dimples are pressed.” Sarazen explained: “Most FMJ bullets break along the cannelure and then fragment into two or three large pieces. With the dimpled bullets, you have multiple fragmentation points so the bullet literally blows up like a grenade in the target.”
Link;
http://accurateshooter.wordpress.com/2009/04/01/us-army-team-tests-radical-new-dimpled-bullet/
