Corbin Rebated Boattail Dies

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Rebated Boattail
Core Swaging	Core Seating	Point Forming
RBT Die Sets	RBT vs BT	Instructions

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.

A conventional boattail bullet tends to focus the 10,000 fps muzzle gas into a ball in front of the emerging bullet, just like a water hose nozzle focuses the water into a stream. Then the conventional boattail bullet must pass through the turbulent ball of gas, and is buffeted about more than the rebated boattail, which passes through clear space as a ring of gas expands outward from the rebate step on the back of the bullet. (For a technical study of the RBT versus the BT base, log onto www.Swage.com and download the technical paper on rebated boattail bullets by Dr.Fitzgerald).

Any FJFB-3 die set can be made into a RBT as well as a flat base, just by adding the RBT-2 kit to it. The RBT-2 kit is a set of parts that seats the lead core in two steps. First, it preforms the base of the bullet in a boattailed shape, and then in your core seating die with a special precisely fitted punch replacing the conventional flat base punch, it forms the RBT base while you seat the core a second time.

When you add the RBT-2 die set to the FJFB-3 die set, you can make both flat base and RBT bullets. This package is what we call the FRBO-5 set, since it has the ability to make both flat and RBT bullets. But if you leave out the flat base parts, you will have the RBTO-4 die set, which only makes the RBT base bullets and not flat base. The difference is the CS-1 core seating die. You only use the core seating die to make flat base bullets. When making RBT bases, you set aside and do not use the CS-1 die.

The procedure for making a RBT base bullet is:

Swage the core in the CSW-1 core swage die. This gives you an accurate weight and proper diameter to fit the particular jacket.
Seat the core in the regular flat based jacket, in the BT-1 die. This is the first die in the RBT-2 die set. It puts an angle on the bullet base, usually from 9 to 12 degrees. A conventional external core seat punch is used (which fits the jacket ID).
Seat the core again in the BT-2 die, which expands the angled base outward to form a concentric step between the base angle and the full diameter shank. The same core seating punch (external) is used for both the BT-1 and the BT-2 dies.
Form the ogive (point) on the bullet using a PF-1 point form die, but instead of a flat base external punch, use the RBT external punch. It has a cavity shaped like the boattail, which supports the base and sharpens the edge.

The rebated step is usually somewhat rounded after the BT-2 operation. It acquires its final sharpness in the point forming operation. Attempts to make the base step too sharp by applying more pressure than necessary can break the BT-2 die, or separate the jacket from the hydraulic effect of the core extruding forward. Also, lubricant build-up in the die, at the rebate step, can prevent the formation of a reasonable step in the bullet. Periodically use a cotton swab to wipe out excess lube if the step does not appear to be forming well enough.

A particular problem can occur with certain jacket materials, such as copper tubing or certain alloys of drawn jackets, which may require an adjustable shoulder core seating punch. The problem is flow of the jacket material, elongating the jacket, when the core is seated. This can give you random length changes in the bullet even though the weight is consistent. The tip diameter appears to vary from one bullet to the next. In extreme cases, the jacket can actually separate at the rebate step from excessive stretching (actually, it is plastic flow of the jacket material from the core pressure).

The solution is to use an optional shouldered punch to seat the core. A punch with a caliber-sized sleeve or enlarged diameter that comes against the end of the jacket when the core is seated will prevent any forward flow of the jacket. The punch can be made for one specific jacket wall thickness and seating depth (a fixed position shoulder). But to allow adjustment of the core length (and weight), the punch can be made like a micrometer, with a threaded sleeve or barrel, so that the shoulder position is adjustable.

With copper tubing jackets, as well as some alloys of drawn strip jackets, it is advisable to include the PUNCH-HA (adjustable type -H, .375 diam or smaller), PUNCH-HL (adjustable type -H, larger than .375 diameter), PUNCH-SA (adjustable type -S, .375 or smaller), or PUNCH-SL (adjustable type -S, larger than .375) with your order for RBT dies.

RBT DIE SETS

The following die sets make RBT base bullets:

RBT-2-S or RBT-2-H
Add to any flat base die set for the capability of making RBT base bullets. Includes:
1. BT-1 Boattail preforming die
2. BT-2 Boattail rebate finishing die
3. External core seating punch (to fit a specific jacket)
4. External RBT point forming punch (use in your PF die)
RBTO-4-S or RBTO-4-H
Rebated boattail 4-die set, makes open tip RBT bullets. Same as a FJFB-3 die set without the CS-1 core seater, but with the RBT-2 die set. Includes:
1. CSW-1 core swage die to adjust the weight
2. BT-1 Boattail preforming die
3. BT-2 Boattail rebate finishing die
4. External core seating punch
5. External RBT point forming punch for PF die
6. PF-1 point forming die
FRBO-5-S or FRBO-5-H
Add a CS-1 flat base core seater to the above 4-die set, and you have the 5-die flat or RBT open tip die set. You can add the CS-1 any time, yourself, or order this catalog number to get it all in one package. Includes:
1. CSW-1 core swage die to adjust the weight
2. CS-1 Flat Base core seating die (not used for RBT)
3. BT-1 Boattail preforming die
4. BT-2 Boattail rebate finishing die
5. External core seating punch
6. External RBT point forming punch for PF die
7. PF-1 point forming die
RBTL-5-S or RBTL-5-H
Add a LT-1 lead tip former to the RBTO-4 4-die set, and you have the 5-die RBT lead tip die set. The set makes both open tip and lead tip bullets. You can add the LT-1 any time, yourself, or order this catalog number to get it all in one package. Includes:
1. CSW-1 core swage die to adjust the weight
2. BT-1 Boattail preforming die
3. BT-2 Boattail rebate finishing die
4. External core seating punch
5. External RBT point forming punch for PF die
6. PF-1 point forming die
7. LT-1 Lead tip shaping die w/RBT base punch
FRBL-6-S or FRBL-6-H
The 6-die set does it all. It makes flat base, RBT base, open tip, and lead tip bullets. The dies you use depend on the bullet you want to make, and you never use all of them on the same bullet. You can use 3 dies (CSW-1, CS-1, and PF-1) to make flat base open tips, for instance. Includes:
1. CSW-1 core swage die to adjust the weight
2. CS-1 Flat Base Core Seating die
3. BT-1 Boattail preforming die
4. BT-2 Boattail rebate finishing die
5. External core seating punch (works with above 3 dies)
6. External RBT point forming punch for PF die
7. PF-1 point forming die
8. LT-1 Lead tip shaping die w/RBT base punchd

The RBT base is especially useful for subsonic bullets, and for bullets fired over very long ranges where even a slight improvement in the BC can have a cumulative effect on drop. It is least useful for bullets fired at short to medium ranges at high velocity, because most of the drag is on the nose of a supersonic bullet rather than the base. When the velocity drops below the speed of sound (which depends on atmospheric density and thus varies with the weather and altitude but is around 1130 fps), the drag caused by the base turbulence becomes a much larger percentage of total drag. In that case, a rebated boattail can sometimes make as much as a 40% improvement in the BC.

This high gain factor seems to apply best to bullets which do not usually require it, such as handgun bullets. A test of .45 ACP bullets, with exactly the same nose shape, weight, loads, and construction, but with flat base, conventional boattail base, and rebated boattail base, fired from the same guns, was conducted by DR Corbin in 1979, and the bullets were recovered and macrophotographed to examine the rifling. The flat base and rebated boattail base had no signs of copper melting on the edges of the rifling, and the boattail bullets had visible droplets of copper from gas escaping past the boattail by compression of the base. The boattail and rebated boattail bullets both chronographed higher velocities at the target and had significantly higher BC than the flat base. A full article may be seen in the Corbin Technical Bulletins, Volume III. (Summary: the RBT delivers significantly higher BC than the flat base, slightly lower BC than the standard boattail, and has modestly higher accuracy than the standard boattail with less copper fouling caused by gas cutting along the rifling of the bullet in conventional boattails, and somewhat less barrel wear.)

Why Doesn't Everyone Use RBT?
(Refer to Dr. A.B. Bailey, "An Aerodynamic Study of the Lapua Step Boat-tail Rifle Bullet and its Ballistics", 1981, Endrickvale, Fintry by Glasgow, Scotland, to R.H. Kent, "The Theory of the Motion of a Bullet About Its Center of Gravity in Dense Media, with Applications to Bullet Design", January 1957, Ballistic Research Laboratories, Aberdeen Proving Ground, Maryland, USA, and to D.L. Walters, "Crosswind Weighting Functions for Direct-Fire Projectiles" (Report X-65), August 1975, Atmospheric Sciences Laboratory, US Army Electronics Command, White Sands Missile Range, New Mexico, USA, for background and mathematical support of the summary presented here.)

A common question is, "If the RBT is so good, why don't the mass producers make it?". This is a good question and deserves a thorough answer. The technical advantages of the RBT do not always overcome the marketing and psychological factors in favor of conventional boattail design.

Rebated boattail bullets have a number of technical advantages over both flat base and standard boattail base bullets. 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 perpindicular 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.

In short, the RBT base preserves the benefits of a flat base for internal and transitional ballistic effects, while providing substantially the same external ballistics for a low drag flight. It is also easier to make precisely, without spending as much on tooling as the same quality of conventional boattail. The main reason it is not used more often in mass production is that the conventional design is well entrenched, the equipment and advertising has long been amortized in regard to convincing buyers about the advantages over flat base designs, and the angled boattail base is easier to load in automatic equipment than either a flat base or a rebated boattail (either of which can be loaded easily if the case neck is slightly belled and properly sized, especially with a straight-line seater). The average buyer of mass produced bullets would not be easily (cheaply) convinced that there was enough advantage for his kind of shooting for a large company to convert their tooling to the RBT over the conventional BT.

However, a shooter advanced enough to make his or her own bullets generally is aware of technical advantages to a much higher degree than a buyer of mass produced bullets. By far, the greatest percentage of custom bullet makers build and shoot RBT base projectiles compared to conventional boattail, because (1) they can see the advantages and (2) they are able to take advantage of them. It does one no good to see the advantages if one has no way to get the bullets that have them. At one point, the Lapua organization in Finland sold their highly respected RBT bullets through a major US bullet firm, but the cost of importing and transportation halted sales after a short time. During the time they were available, they were in great demand among long range competitors and provided extremely fine results.

Making a RBT

The RBT base is made by seating the core into a standard flat-based jacket using two dies. The first die is the BT-1, which forms an angle on the base using the pressure on the lead core to push the base into the angled die cavity. The second die is the BT-2, which accepts the angled base from the first die, and pushes the jacket straight out to form the rebate step in the jacket. The same external punch is used in both dies. It is the same punch that would be used in the regular flat base core seater.

This 2-step method of forming the RBT base insures a precise, even step. The final sharpness of the step or rebate is made with a punch, when the ogive is formed in the PF-1 die. This special punch has a cavity machined in the end, which matches the angle and depth of the boattail. This punch should always be used with the proper RBT base seated cores, and inserted a full caliber of depth into the point form die before any pressure is applied. Attempting to make too long a bullet for the die would place the unsupported cavity of the punch outside the die. The internal pressure generated within the bullet would stress and crack the wall of the RBT punch. Likewise, using the punch in the wrong die could result in pressure cracking the wall of the punch. The punch must be snugly supported by the die when pressure is being generated.

If you have a FJFB-3 die set (three dies, for flat base) and add the RBT-2 die set to it, you would then have a five-die set capable of making either flat or RBT base bullets. The RBT-2 die set can be added to any flat base set, including the LTFB-4 lead tip set, which would result in a six-die set capable of open tip, lead tip, flat base, and RBT base bullet making. This is sold as a single package called the FRBL-6 die set (either -S or -H is added, to indicate which kind of press it fits).

If you did NOT want to make flat base bullets, you could order a RBTO-4 die set, which would leave out the CS-1 core seating die. The lead core is always seated directly into the BT-1 die, not first seated in a flat base die. Therefore, the CS-1 core seater die is not required if you do not want to make flat base bullets. If you wanted to make either open tip or lead tip bullets, you could add the LT-1 lead tip die, and have the RBTL-5 die set. This consists of the core swage, the RBT-2 two-die set, the point former, and the lead tip forming die. You can see that the various catalog numbers are just shorthand designations for the combination of basic dies to make the kind of bullet you want. You can either buy them as sets, or add dies to add more capability to your existing set.

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