Conventional boattail vs. rebated boattail

[pogo]

I understand Sierra tested the rebated boattail and found it inferior to the conventional boattail, and Corbin in "ReDiscover Swaging" recommends the rebated variety due to the possibility of damaging the punch.

Factories don't have that problem of damaging dies or punches, it seems.

Is there a consensus as to which really is better for shooting?

[Lead pot]

Hi Pogo,
I found the flat base .308 's I make shoot just as good as the boat tail or rebated boat tail with in 200 yds.but the bt.and rebated bt smile past 300 yds.

I run the 168 gr sierra bt match through my dies and put a rebate on them,and tested them at 500 yds,the groupe was the same size with boath.What I found interesting was the rebated boatail impacted 1 3/4"higher.Lp.

[Donna]

The boattail design really decrease drag when a bullet is in the transonic flight. The boattail does reduce drag in all stages of flight but it’s in the transonic stage where the greatest drag forces are applied to a bullet. The transonic velocity is were the bullet is decelerating from supersonic velocities down to subsonic velocities, between about 1250 fps to 1050 fps.

Lead Pot that is why the boattail or rebated boattail did better than the flat base at longer ranges.

I don’t know about Sierra testing the rebated boattail design. But Mr. Dave Corbin writes in his swaging books, that he recommends the rebated boattail design for a number of reasons. It simplifies the design of swaging dies, to use the rebated boattail design rather than the conventional boattail. It has been proven mathematically and experimentally that the rebated boattail design is superior to the conventional boattail: it imparts more energy, in velocity, to the bullet on exiting the muzzle; it allows the bullet to fly through undisturbed air after exiting the muzzle because the hot gasses are deflected out to almost right angles to the bullet’s flight instead of flowing up passed and into the flight path of the bullet; and because of the step from the boattail to the shank of the bullet it makes a 90 degree angle to the bore when fired, this 90 degree angle greatly reduces the hot gasses ability to flame cut the riflings.

The factories don’t have the problem of damaging the punch or dies because they make their bullets in a slightly different way than we do. :eek:

Lapua is using the rebated boattail on some of their 30 caliber bullets now. :wink:

Donna

[pogo]

Thanks folks - interesting stuff.  I thought I read a blurb some time ago that Sierra tested RBT vs conventional and saw a 25% accuracy loss vs. conventional.  I don't recall a particular reason.

I don't shoot past 600 yards so the point really is academic.    Store bought bulk is just as good as the best match boattail for highpower.  

I swage .224 bullets for .223 Remington, and .288 bullets for a 280 ross.

[pjh421]

Pogo,

One rifle may shoot RBTs better and the next might shoot conventionals better.  Also, Sierra is in the business of selling conventionals.

Paul

[Donna]

Hello all,

The factory bullet makers don’t always make or incorporate the best designs in their bullets. For one the conventional boattail design, the factories bullet makers has popularized the 9 degree boattail angle to the shank but in actuality a 7 degree boattail angle to the shank is the most efficient in reducing the drag coefficient.

Reference:

1) Karpov, B.G., “The Effect of Various Boattail Shapes on Base Pressure and Other Aerodynamic Characteristics of a 7-Caliber Long Body of Revolution at Mack = 1.70,” Ballistic Research Laboratories Report No. 1295, 1965.

2) Kneubuhl, B. P., “Optimization of Boattail for Small Arms Bullets,” Defense Procurement Group 2, Ballistics Division, Ministry of Defense, Switzerland. (Presented at the International Symposium on Small Arms at Quantico, Virginia, 1983).

So it is not hard to understand why the factories are still making bullets with the conventional boattail, their factories are; 1) already setup for the conventional boattail, and 2) their present manufacturing process makes the conventional boattail design easy and economical to make.

Donna :wink:

[jsmerc01]

Donna, I don't have the calculus to follow along with the citations you gave.  I guess that's why it's a one way bridge from the engineering school to the business school and finally to the poly sci deparments, eh? :wink:  

I'm especially interested in bullets for use in matches that run from 600 to 1000 yards.  I have read Mr. Corbin's swaging handbook and paid particular attention to the part where he discusses the advantages of a rebated boattail design versus a conventional boattail.  It's especially obvious  that he feels machining a set of dies with a rebated base is much easier to accomplish.  

My question is this:  Are you satisfied that the proof offered theoretically jibes with what happens in the real world in regard to improved BC?  Further, would you care to estimate the improvement offered for firing a 6.5 caliber BTHP with both boattail designs at 260 Remington or 6.5/284 velocities at 1,000 yards?  You stated that it's in the transonic range that the improvements are most noticeable and since long range shooters have a goal of keeping their bullets supersonic all the way to the target how important is boattail design for this type of shooter?  My guess is that the elevation difference would be still be caught in the X-ring and certainly not measurable with my very ordinary shooting abilities  :oops:

Thanks for your insight!

Scott

[Cat Whisperer]

Donna -

It is most professional of you to list the references!  It raises the quality of the posts by referring to original sources.  (I'm going to have to brush up on my calc. to go through the proofs) but to read through and pick off the conclusions should be enlightening.

Most of us go through life guessing.  Taking the experience of others as starting points and adding our more or less good 'experiments'.  The sources you are brining to our attention change for the better our starting points.

[Donna]

Hello jsmerc01,

In one way it is easier from a machining point of view and in another way it is not. It takes two dies to make the rebated boattail vice one die for a conventional boattail. The punch on the conventional boattail that pushes it into the point-forming die has to have a knife-edge for it not to leave a mark on the bullet and this knife-edge brakes often whereas for the rebated boattail the punch is quite hardy. There is two ways of making a rebated boattail that I know of; one is to make a punch with the design in it, like Dave Corbin does, which is an easier way but is not very hardy, and the other way it to make the rebated boattail design in to the die itself, like Richard Corbin does, which is in my opinion the best way.

I am very satisfied that the rebated boattail’s performance both theoretically and in the field is superior to that of a conventional boattail. Now let me say this, the difference of improvement is small but when you’re going out to 1,000 yards or more any small advantage can be significant. The transonic zone is where the improvement of the rebated boattail is most noticeable but it is also very noticeable in the subsonic range too. The quest is to keep the bullet in the supersonic zone as long as possible but at 1000 yards most of the time will be spent in the transonic and subsonic zone for most of the range and a boattail or rebated boattail design is very important. The longer the range the more significant the boattail or rebated boattail is to a bullet’s performance. Some people do not think a small improvement is worth their time or money but that is up to the individual. But I do know this, when you start adding up a small improvement here and there pretty soon all these small improvements add up to be quite significant and as you increase the range at which you are shooting small improvements are multiplied by a factor rather than a coefficient. :eek:

Cat Whisperer,

Thank you. :roll:

Donna :wink:

[jsmerc01]

Hi Donna, et al., what a thread we have here, very thoughtful discussion going on eh?

Here's what I have in mind.  I shoot two, perhaps three disciplines depending upon how you count: short range (100 & 200 yards) benchrest (score and group), high power x-course and long range (1,000 yards) and I'm contemplating several sets of bullet dies.  Two sets would be for high BC 6mm and 6.5mm bullets, another set would be for lightweight 30 cal bullets that would be used in short range benchrest shooting.

My questions surround the long range bullet design discussion that will go on with a die maker soon.  What elements does a modern long range bullet incorporate for a highest feasible BC without resorting to exotic materials, complicated procedures for making the bullet itself and keeping the price of the dies reasonable.

Is there a difference in bullet design based upon caliber?  I've done some work as a commercial pilot and am aware of certain properties in aerodynamics such as drag increasing very rapidly (squaring) as velocity increases (doubles).  Drag also goes up very quickly as "wetted area" of an aircraft increases.  Does this apply to bullets as well when we are talking about a 20 thousandth of an inch increase in diameter.

thanks for the help,

Scott

[talon]

Scott, please jump to   www.corbins.com/bullets.htm#uld  for background until Donna &/or others join. I take it you are only concerned with these 3 calibers and only for paper target shooting ( this will narrow the discussion)  

[Donna]

A lot of what you are asking for has already been addressed on this forum. As I have said before an Aeronautical engineer is not an Aeroballistic engineer and the reveres is also true. As you have said Jsmerc01, the drag does increase to almost the square of the velocity but that only hold true for velocities just below the speed of sound, after that things start to change. And there is no such thing as common sense, but that is another topic. And yes, the drag increases as the diameter increases. There are two ends of the bullet that needs to be addressed, the front or nose and the rear or base. The nose is what design should the curvature be and what is the meplat diameter? No one answer is 100% correct. There is give and take with everything. This is because usually the bullet travels from supersonic to transonic and into subsonic velocity zones and each one of the three has certain characteristics and therefore requires certain properties for whatever the desired effects you want. One of the most important factor in Bullet Making 101 is density, getting the most weight in the smallest package that is why we use the purest lead because it has the most weight per volume at the best price. If you can double the weight for the same cross sectional area you will almost double your BC alone. By now you might be thinking that I’m saying a lot of nothing, well your almost right. When I think a person wants something handed to them on a silver platter for nothing I am less likely to want to help. There is a lot of information already put out on this forum. I will however be glad to further explain anything that you don’t understand.

Donna