A Question for Warren

Blaine Fields

Well-Known Member
Sep 10, 2001
San Jose, CA
I've recently read an explanation of the high speed gas aerodynamics surrounding the exit of a rebated boat-tail at the muzzle. Apparently the claim is both in and at the muzzle, the bullet performs more like a flat bottomed bullet, but has the advantages of the boat-tail in flight. If you have an opinion about this bullet configuration I would like to hear it.

A rebated boattail does not have the aerodynamic advantages in flight of a true boattail. These advantages are most evident at about Mach 1.06 and below, but shadowgraphs of supersonic projectiles clearly show that dramatic breakpoints in the airflow at the heal cause amplification of the resulting shockwave and potentially increased drag. You can also see these at cannelures.

The argument over which design gives the better seal is moot. All "inherent advantages" are functionally driven by the materials of construction and the degree of precision that the design is executed. Specifically, the "better seal" assumes a certain degree of base upset which varies considerably with the materials of construction.

There are a multitude of claims about the nature of the gas flow around the bullet as it is exiting the muzzle. I have to admit that this is an area in which I am learning quite a bit recently. We have been working with Phil Seeburger of OPS Inc. on new suppressors. Phil is the master of muzzle gases. A lot of what you read about this exiting hot air is just that. About the only categorical thing I can say is that the gas has to be directed symmetrically from the muzzle and off the base,heal, boattail of the bullet or the rear of the bullet will be "kicked" to one side. Small imperfections can make a big difference. As to whether there is an inherent advantage of one form or the other in directing these gases and allowing the bullet to yaw less or settle down sooner I am not sure it exists. Precise measurement of these effects is not easy and proof of one or the other's advantage tends to take the form of accuracy results at some distance downrange. These results can be interesting but as we all know could be the result of some entirely different factor. I guess what I am saying is that I am yet to be convinced of any specific advantage at the muzzle, but as I said earlier I am learning fast in this area.

Do you have a specific idea that you favor?

It is my opinion that rebated boattails exist primarily for manufacturing reasons not for ballistic reasons.

[ 09-11-2001: Message edited by: Warren Jensen ]

[ 09-11-2001: Message edited by: Warren Jensen ]
No, I'm just trying to puzzle this out.

The explanation that I read was that gas would flow, almost in a laminar fashion, around a boat-tailed bullet and create a fireball in front of the bullet which the bullet would have to traverse, increasing its upset. The representation was that the rebated boat-tail tended to interrupt the gas flow at its rebated edge thereby reducing bullet upset.

Prior to reading this short explanation, I had never heard of a rebated boat-tail. After learning of its existence, I had doubts about its superiority based upon that explanation which is why I sought your views.

In terms of my own intuitive analysis this is what I came up with. As the bullet moves down the barrel, it certainly is compressing the air column in the barrel and pushing this air column forward. Whether the bullet ever reaches supersonic status relative to that column of air, I am not sure, but I think that I may not although the air column probably becomes supersonic. As the bullet begins to exit, it is still in this moving air column and therefore may not yet be supersonic. As the body of the bullet at the beginning of the boat-tail taper begins to exit, supersonic gas escapes around the bullet and bullet upset begins. Then, the expanding gas moves by and around the bullet so the bullet is now traveling possibly at a supersonic velocity base first relative to the expanding gas, a very stable, although drag laden, aerodynamic configuration. As the gas slows and the bullet leaves the supersonic envelope of gas, it hits essentially stagnate air and develops shockwaves at the normal locations.

So, just thinking intuitively, I would guess that laminar flow of the expanding gases around the exiting bullet would have a stabilizing effect and that to the extent that a rebate interrupts that flow and increases turbulence around the already upset bullet. the effect would not be helpful and possibly harmful to accuracy. So my guess would be that the normal boat-tail would stabilize sooner that the rebated boat-tail. As mentioned, however, this is just a pure guess.

It is 0620 hrs, Wed. 9/12/01. Yesterday was a bad day. All of our lives in one form or another will be changed permanently.

Back to the muzzle gas flow question. There are two things we know for sure. One is that the gas just in front of the bullet as it nears the muzzle is traveling at nearly the same speed as the bullet. Two is that at some distance of inches, maybe 2"-10", the front of the bullet will encounter clean, undisturbed air and normal flight airflow and shockwaves will ensue. The question is what happens in between these two events. I believe that a large number of different things can and does occur with different loads, bullets, pressures, velocities, and muzzles.

I am only discussing muzzles that are not vented, braked, suppressed, or the vent gases mechanically stripped.

As the bullet exits the muzzle and the seal is broken the pressure can vary from 1000-8000 psi., depending upon the load and pressure curve. This gas will then accelerate past the base of the bullet. The bullet is transitioning from rotating around the center of form dictated to it by the bore and grooves to rotating around it's center of mass. There will always be some amount of yaw and wobble here causing the base to offset from bore centerline. As the gas is accelerating past it will amplify this offset from a little to a lot. This is the muzzle jump wobble that you hear so much about and it is caused by events at the rear of the bullet, primarily, not the front. As the gas accelerates past the bullet it's pressure and velocity is dissipating very rapidly. In short, round nosed bullets from loads with very high vent gas pressures the gases can disturb the air in front of the bullet causing turbulence. Short round nosed bullets will have the highest Stability Factors and will be the least effected by turbulence. With long ogived bullets from loads with lower vent gas pressures I do not believe the gases measureably disturb the air in front of the bullet's nose. They are deflected to the side at some angle and any envelopment around the nose would be so dissipated as to be of little effect. The concept that this would be laminar flow is incorrect, in my opinion. The bullet is gyroscopically stabilized and it is the oncoming clean air that introduces the overturning moment. So until it encounters clean air the bullet has little destabilizing forces acting on it from the front. It is the rear where the cause of the yaw and wobble is initiated.

As for there being a fireball that the bullet flies through, I doubt it. That would require burning or unburnt powder to accelerate past the base to the nose and I am very dubious of any circumstances in which this would occur.

[ 09-12-2001: Message edited by: Warren Jensen ]

[ 09-12-2001: Message edited by: Warren Jensen ]
Regarding the quality of the air flow around the bullet as it exits: a .308 exiting at 2700 fps is traveling at around Mach 2.3. The exiting gas is traveling at more than Mach 1 faster, correct? If so, wouldn't shockwaves form at the leading edge of the base producing subsonic flow behind and over the surface of the bullet? And wouldn't this flow be essentially laminar? I'm asking because I don't know. I've never seen shadow graphs of a bullet with the base leading, but I would presume that the air would pile up in front of the base and that a stream of moving air would seperate in front of the base, moving around this air mass, pick up speed (venturi effect) and form a standing shockwave at the edge of the base and another where the boat-tail ends and the body of the bullet begins. So I am assuming that the flow on a backwards bullet is essentially laminar all the way around it.

I am basing this on shadow graphs of normally oriented bullets, where the flow is apparently laminar until it hits the base where is becomes turbulent. My intuition is that this is what the spotter sees as the bullet wake.

At any rate, getting back to the rebated edge, wouldn't the proper way to evaluate this configuration be based upon whether it enhances or is detrimental to the flow properties of the exiting gas around the base of the bullet at the muzzle? Or have I completely missed the boat here?

Please explain your Mach 1 number. Remember, we are speaking of an environment of several hundred if not thousands of pounds per square inch and temperatures of greater than 1000 deg. F.

The leading shockwave on a flatbase bullet, flying backwards at supersonic velocties would be way out in front of the bullet.

As to whether to evaluate a rebated design on whether it is superior or not in venting or controlling gas flow in the first few inches after the bullet leaves the muzzle is fine. Remember, it is demonstrably not superior after you get 10" downrange. A rebated designed will increase drag, at supersonic and subsonic speeds, and will add instability at transonic velocities.
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
Please explain your Mach 1 number. Remember, we are speaking of an environment of several hundred if not thousands of pounds per square inch and temperatures of greater than 1000 deg. F.

I see your point and I don't know the answer.

A big difficulty in making calculations concerning the muzzle gas flow is the extreme transient nature of the parameters. The gas pressure goes from many thousands of psi to zero in a fraction of a second and the temperature changes over a thousand degress F. in nearly as short a time.

The first shock wave of the gas exhaust occurs at the crown of the muzzle. This is a true supersonic nozzle shock and is reflected back into the center of the stream. With a flat based bullet the reflected shock will encounter the bullet base as it is leaving the muzzle. With a boattail it will encounter the heel, or boattail and be reflected off. As the bullet goes further away from the muzzle there indeed may be a point where the delta V of the gas over the bullet heel may be greater than 1200 fps but in that environment it will take a much great delta V than that to create a shock. Collected data does not support the theory as the recorded noise would have to be different, and it is not.

Again, the gas flow amplifies the initial yaw caused as the bullet transitions from rotating around it's center of form to rotating around it's center of mass. This assumes symmetrical flow initially. Any nonsymmetries in muzzle, crown, or bullet base will add other yaw vectors.

A well designed bullet has yaw damping factors in the negative numbers to reduce the yaw. Any bullet that doesn't will become unstable in short order.

[ 09-19-2001: Message edited by: Warren Jensen ]

[ 09-19-2001: Message edited by: Warren Jensen ]
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