ALCO Rebated Boattail Bullet ULD Design for High-Performance Bullets

Posted by on 3/9/2019



ALCO Engineered Excellence
Rebated Boattail Bullet Design

ALCO Bullets uses a Rebated Boattail (RBT) design for all of our ultra-long distance (ULD) projectiles to give you an edge with increased accuracy when you are shooting long distances. It is just one factor that adds to our bullet’s success and produces favorable results for those who use them. In short, the RBT was initially created to prevent the muzzle gas from becoming a ball as it exits the gun which causes turbulence for the bullet. This is what happens with a flat base bullet and those with a conventional boattail (BT). The advanced RBT design, will deflect the muzzle gas causing it to flow into a ring with a center, allowing the bullet to have a clear path to pass through smoothly. It also provides an enhanced seal in the boar causing a rapid and concentric transition to free flight as the bullet emerges from the barrel. All of these things add to accurate trajectory and increased bullet performance.

ALCO feels utilizing a rebated boattail is an important element for our premium bullets. It takes additional press time for each projectile to incorporate the RBT base. This significant detail ensures the quality handmade precision bullets you expect.

REBATED BOATTAIL DESIGN
RBT Overview from Corbin Manufacturing

“The purpose of the boattail is to reduce air turbulence at the bullet base. In theory, a double-ended bullet would be even better at achieving this, except that it would in most cases make the bullet too long and take up too much powder space in the cartridge. Truncating (cutting off) the base at a somewhat smaller diameter than the bullet caliber reduces base drag enough to make a noticeable difference in performance at longer ranges. 

A RBT or Rebated Boattail base is an advanced form of the conventional boattail base, which adds a 90-degree step or shoulder between the angled base and the shank of the bullet, to act as a "spoiler" and break up muzzle gas flow, provide a better seal in the bore, and cause a more rapid and concentric transition from the muzzle to free flight than a boattail angle emerging from the barrel.

There is approximately 15% improvement in the group size, on average, with identical bullets fired from the same gun, in favor of the RBT over the standard boattail. This is brought about by the improved gas flow pattern at the muzzle, as the muzzle gas impinges on the angled rebate and is blown off in a circle of expanding gas. The conventional boattail tends to focus the muzzle blast in front of the bullet, similar to the focus rod in a fire hose nozzle. Laminar flow of gas around the emerging bullet follows the bullet outline until it breaks up just ahead of the bullet, causing turbulence in the bullet path. This is greatly reduced or eliminated with both flat base and rebated boattail bases. 

Bore life is generally longer when shooting RBT base bullets as compared to conventional BT bases, because of the superior gas seal made by the rebate or step as compared to the angle on the junction of the boattail with the shank. Gas pressure acts "normal" or at 90 degrees to the surface of its container, vectoring force perpendicular to the surface. The force on the boattail angle tends to "peel" it back away from the rifling contact and force gas past the bullet, whereas the gas pressure on the rebate angle acts parallel to the bore, and does not have as much of a wedge effect. 

The small penalty in base drag created by the rebate is more than compensated by the gain in group tightness caused by the muzzle gas effects, and the rebate is more easily formed in a precise, concentric ring than the sloped junction of a 12 to 15-degree boattail angle. (Some conventional boattails can be shown to have one size slightly higher than the other, so that gas escapes first on the shorter side as the bullet emerges.) 

Finally, the release of gas is more abrupt at the muzzle, rather like popping a cork instead of sliding a tapered plug from a bottle, giving the escaping gas a little less time to push on one side or the other of the bullet as it first emerges. The less time spent in this partially-contained gas state, the more likely it is that the bullet will be launched without tipping from uneven release of muzzle gas. The conventional flat base is similar in providing a quick release compared to a conventional boattail.”