.308 cal 200 gr. BBT Whiteout

[rkr]

Why not use a technique that's available for all of us, cast bullets, sized to land diameter or slightly over and then knurled? Or is there something I'm missing in this thought?

Interesting thought, I do not know enough about the effects of knurling on a bullet to comment on how well it would work.

It does seem to me that a mold would be much easier to make if there were no driving bands.

One could very easily make a jig to go between center on a lathe to knurl a few bullets for testing.

Or we could just take a standard mold, size the bullet to land diameter of an individual rifle and then knurl a rear band to seal it to the bore. Since we have lots of variance in land diameters this would allow easy tailoring or bore riding (or is it land riding?) bullets for each barrel.

[rsterne]

Exactly the technique I am suggesting.... and that Bill G is set up to use for the swaged bullets he sells.... He would, however, have to make the dies for the nose and boattail, but would have the ability to change the length and weight, of course.... The advantage to knurling a band, or bands, is that you can tailor the number and width of them to prevent stripping in the fast rifling when being kicked by 4500 psi of Helium....

"Land riding" is I guess the correct term for bore-riding, but that is commonly used, and I suggest that it should JUST have the slightest rifling marks on the body (about 0.0005" or so) when pushed through.... The "correct" body size you should be able to push through by hand without difficulty, and yet leave rifling marks on the body....

Bob

[K.O.]

jr460 your reading comprehension skills are about as lacking as your ability to complete a sentence without  a grammatical error. Please read rsterne's post once more and realize he designed this bullet for subsonic use and at no point recommended/suggested it be used at 1500fps. I personally am tired of your post as I feel they add nothing to the thread. This is not because of your lack of understanding of ballistics because it looks as if you have plenty of knowledge in that area but your inability to read rsterne's post and not somehow see something that's not there.

I hope you somehow are able to read back over the post and understand what's actually being posted so that maybe we can learn from what you and rsterne obviously know more about than the rest of us. As of now all I do is read your post and wonder how you managed to put two totally separate facts posted by rsterne together to create something you can disagree with.

Lets connect the dots .
 1 There is no drawing or mention of the same bullet in a flat base version being used to prove the rebated boat tail will do what is being calmed.
2 what rate of twist will be used
3 specifications of barrel; chamber speck, lead speck,lands and groves speck, how many,right hand twist,left hand twist,bore diameter,rifling twist rate, rifling cut,micro grove ,hammer forged or button, air gauged{ MATCH} or standard, LENGTH OF BARREL.
4 at what rpm was this data generated with.
5 How can you have a mould made with out having this data?
6 Most important what lead is this mould being cut for,lead tin,lead tin antimony,lead antimony or pure lead ?
With out these answered the mould and drawing is just a wast of time and money and  just a theory no facts will ever be proven.
Yes I did read THE POST it is just a rambling with no facts to show  how the answer came to be. 
David

once again I can not help but wonder about your tone...

Why not try to  explain with your knowledge what would have the best chance of success and the problem still faced at that point and then try to come up with  solutions...

It would be much more productive , in my mind at least...

[oldpro]

I agree! Useful data even if it doesn't work is better than no data. I have no idea if the boat tails work or not but the data can be used to either make a working prototype or move in a new direction. Bob keep up the hard work your ideas and math to back them up is fascinating and I for one would love to test it out in a real world environment.

[rsterne]

The comment has been made that the models I have been using are for conventional boattails, and not rebated boattails, and that is completely correct.... However, Lapua and Corbin have been using rebated boattails very successfully for decades.... Considering that the math models show that the drag for conventional boattails have less than half the drag of a flat base bullet when subsonic, even if the rebated boattail is 25% more drag than a conventional one (and the BC of the Lapua bullets don't show that), the drag of a rebated boattail would still be a third less than a flat base.... I know from testing (mine and others) that the BC of my existing Bob' Boattails are significantly better than the same weight bullet when subsonic, so I know I'm on the right track.... So far, because of the relatively short bullets I have converted to boattails, I had not had the opportunity to try a "normally proportioned" boattail, which should work much like the Lapua and Corbin bullets when subsonic.... Until somebody bites the bullet (pun intended) and tries it, we won't have the hard data....Until someone shows me hard data that compares a boattail to a flat base bullet of the same nose shape, weight, and caliber (ie the only difference is the boattail) that show the flat base has less drag when subsonic.... I will proceed under the assumption that all the math models, and those that build boattails for subsonic use (eg. Lapua) know what they are doing....

Perhaps it would ease the mind of the person objecting to my efforts if he realized that I don't have any commercial plans, and never have.... All my Bob's Boattail designs are in the public domain, and free for any individual to use, make molds of, etc.etc.... If it is taken up commercially, my only expectation is that the manufacturer ask my permission, and once given, credit me with the design.... I have never asked, or received, any compensation for my design efforts, they are done solely for the purpose of furthering the sport of airgunning....

Bob

[Rizen 1]

In about 45 days, we are all going to know how well these bullets work, what (if any) changes need to be made, and what steps to take next to fulfill Bobs goal of making better airgun bullets.........Because C.Alls, Brent Hoag, and myself are going to be building and testing a gun that will shoot these...    We are also coming out with a t shirt that says.... 

                                                       Words don't put holes
                                                             in the 10 ring....
                                                           
                                                              Bullets do!

                                                      SHUT UP AND SHOOT!

[crosman999]

 Really enjoy reading this thread,wondering why nobody has mentioned johninthecamper? He has two Corbin presses and does very fine work

[CHUCKSTER]

It is a scientifically proven fact that most of the drag is on the nose of a supersonic bullet, while most of the drag on a sub-sonic bullet is on the base. So if we can reduce the drag on the base of our sub-sonic airgun projectiles, it stands to reason that they will fly farther, more accurately, and with less wind drift.

A good read by Dave Corbin is titled: "A comparison of the Rebated Boattail Bullet Design with Flat base and conventional Boattail Bases..." Here are a couple excerpts below:

'The boattail reduces base drag at all velocities, but has the greatest percentage of effect when the bullet is flying at sub-sonic velocity.'

'A 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 ...muzzle gas into a ball in front of the emerging bullet, just like a water hose nozzle focused 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.'

'Hyper-velocity gas escaping around the bullet base at the muzzle will impinge upon the rebate shoulder, which acts as a "spoiler" and deflects the gas from a laminar pattern. It causes a ring of gas to be blown off in a turbulent forward rolling expansion, but leaves a clear space directly in front to the bullet. This eliminates the buffeting of the bullet by its own muzzle gas, a source of up to 15% of the total dispersion factor.'

I understand this to mean the bullet flies through the doughnut hole of quiet air instead of flying through a ball of violently turbulent gas...and if a rebated boattail has the potential to give me 15% better accuracy than a flat base bullet, then that's the one I want to be shooting. The other big advantage of the rebated boattail bullet base  design, especially when it comes to cast bullets, is that the bullets consistently leave the barrel the same way each time. (consistency is accuracy). Whereas the Achilles heel of all cast bullets is the variations of the flat bases from bullet to bullet. No matter how perfect a bullet mold, if the base of the bullets it produces are all slightly different due to the casting process, the bullets will never be as accurate as swaged bullets produced in a press. This is why it is so important for cast bullets to have a rebated boattail, regardless of whether they are being produced to fly super or sub sonic.

It is just entirely amazing to me that Bob Sterne hit upon this idea for his cast bullet designs. I think Bob is laying the foundation for some astounding accuracy out of airguns with his bullets, versus just shooting drag stabilized diabolo shaped skirted pellets or cast flat base bullets. Keep up the great work you are doing Mr. Sterne...can't wait to see what you are producing a year or two down the road...

(For a technical study of the RBT versus the regular BT base, log onto www.Swage.com and download the technical paper on rebated boattail bullets by Dr. Fitzgerald).

Fond regards, Chuckster

[rsterne]

Chuckster.... thank you for posting the info from the Corbin website.... It was one of the main influences in my decision to use the rebated boattail design, as I stated a couple of years ago when I started down this path.... Your independent summary of the reasons is concise, accurate, and exactly on point.... I simply saw the application of a known and proven technology to airguns and have given myself a goal to make it happen....

Bob

[rsterne]

I have had a couple of guys question the BC I quoted from the JBM calculator, and rightly so, as it was higher than the Lapua .308 Subsonic, and that makes no sense.... I guess I should have been clearer that I thought the number was exaggerated, just as it would be for a cast flat base bullet.... This is because the calculators make no allowance for grooves or ridges, drive bands or rebates.... However, the basics still apply.... boattails have a superior BC to a flat base bullet of the same caliber, weight, and nose shape (ie where the boattail is the only difference), particularly in the subsonic velocity range.... Corbin state that the BC of a rebated boattail is 10-15% less supersonic and 40% less subsonic, and I think that is an extremely useful figure to compare to an otherwise similar flat base bullet...

Bob

[K.O.]

Hi Bob often I am not clear as I thought I was...

In fact I think at times because of some probs I am not clear at all...

Heck in my above post about twist I meant a rebated Boat tail and did not make that clear... (in my mind I had )

About the B.C. well it could be very well the method of measurement, because well I just noticed that Midway states it very close to your figures... @

200g subsonic Bt

SD = .301
BC = .382

 so of course it will vary some  also as you have repeated for the novices many times... and until measured it is obviously just as good of an estimate as can be came up with...

[K.O.]

oops the twist comment was the other thread (I think... )

[K.O.]

I just found this Lapua used Doppler for it and so their BC figure probably is not a single speed  B.C. ...


http://www.lapua.com/en/customer-center/lapua-ballistics/lapua-ballistics-software.html

[rsterne]

Lapua is the only company, to my knowledge, that uses Doppler Radar data to develop the exact Drag Coefficient Curve for each bullet.... Since no bullet precisely matches the G1 or G7 model, this is the only method to precisely calculate the trajectory and wind drift as the velocity decays as the bullet travels downrange.... Instead of using the antiquated idea of BC, and quoting a BC that is really only correct at the velocity it was measured, they do it right.... and use the direct measurements of Cd in their calculations.... My hat's off to them for bringing ballistics into the 21st century....

Bob

[Bill G]

Bob and I finally made contact.  Sorry gents, I've been off the grid for a bit for some family time.  This project is extremely interesting but the time line is too short for me to be effective.  IMO, your approach with the cast bullet is the fastest path to your goals.  Now what I could do is knurl some bullets that have been cast and or sized to the lan diameter.  The single driving band at the read of the bearing surface, this will likely make sizing difficult.  I would have to make a knurl for a single band but have one that would do multiple bands ready for use.  If the effort of making such a projectile was taken, I would gladly apply the knurl simply to see it done and tested.  No charge, simply shipping.
That drawing is impressive and I think that a better name for it would be "Javelin".   Over an inch and a quarter,  that thing looks like a miniature scale of the Paris gun shell!! .     

 

[rsterne]

I checked the design with my new McDrag program, and I'm satisfied that the basic design of the bullet is optimized for the job.... Since it matches the G7 Drag Model very closely, I ran it through the JBM Trajectory and Drift Calculator, using several muzzle velocities from Mach 0.8-0.975 to see what happens to the Drop and Drift, and what the Retained Velocity is at 200 yards and 400 yards.... Here are the results....




You can see in both cases that after you go above 1000 fps muzzle velocity, the Wind Drift starts to increase, and the Retained Velocity starts to decrease more rapidly.... The Drop (rifle sighted at 100 yards), however, decreases more rapidly below 1000 fps than above.... Therefore, it makes little sense to shoot below 1000 fps, you don't get less Drift, but you do get more Drop.... Going above that velocity your Drop doesn't change as much, but the Drift starts to increase.... You could undoubtedly push it a little bit, but I wouldn't go above 1050 fps.... Of course shooting is the only way to be sure.... these calculations only let you know what to expect.... To put some hard numbers on the calculated Wind Drift at 200 yards (10 mph crosswind) at various muzzle velocities....

900 fps - 2.2"
1005 fps - 2.4"
1033 fps - 2.8"
1061 fps - 3.2"
1089 fps - 3.9"
1116 fps - 5.5"

Bob

[CHUCKSTER]

Wow Mr. Sterne, it will be interesting to see how close your calculations on paper compare once you actually make some of these bullets and shoot them at 200 and 400 yards...You seem to be as enamored with the mathematical theory as you are with actually helping air gunners find the "magic bullet" for each caliber. It would be fun to have you in the same room with Mr. Dave Corbin for a brain storming session...only God and Werner Von Braun would be able to interpret the results...but I bet some good shooting bullets would be created. I think you are amazing Bob. 
Fond regards, Chuckster

[rsterne]

To tell you the truth, I would be satisfied even if the drag was twice that calculated.... It would still be a huge improvement over a flat base bullet at all velocities up to Mach 0.95....

Bob