Boattails vs Flat Base - a Study

[CHUCKSTER]

Wow Kirby...thanks for your enlightening remarks. You pretty much closed the book on this subject.
Fond regards, Chuckster

[rsterne]

I finally got back to doing some more research on boattail design today.... I studied the relationship between drag and base diameter for four boattail lengths, from 25-100% of the caliber up to Mach 2.... The bullet used was my .308 BBT Whiteout, varying only the boattail, but keeping the weight at 200 gr.... All the calculations were done with the JBM Drag Calculator, but I didn't use the BC, I summed the Drag Coefficient for the Boattail and Base, so a smaller Cd is better.... Here are the results.... 100% diameter (on the right on each graph) is a flat base bullet, there is no boattail....






I know, that is a lot of data, and not the easiest to read, so here is the summary....

1. Below Mach 0.85 (949 fps), the optimum BT base diameter is 50% of the caliber.... Yeah, that is VERY small, hard to cast, even harder to get a mold made....
2. Below Mach 0.85, the drag is the same regardless of the length of the boattail.... so even a short boattail will help significantly....
3. Above Mach 0.9, the longer the boattail is, the less the drag, at any given velocity.... Makes sense I guess, but again, that makes the mold and bullet hard to make....
4. Right at Mach 1, boattails don't have much of an advantage over a flat base, particularly if the boattail is short....
5. Above Mach 1, the optimum base diameter is between 75-95%, depending on the length of the boattail....
6. At supersonic velocities, a short boattail with a small base is a disadvantage over a flat base bullet.... the best shape is a shallow taper....
7. At speeds between Mach 0.85 and Mach 1 (949-1116 fps), there is an optimum base diameter for each length, and the best way to show that is below....

First, the big picture.... Each line is for a different length boattail, measured in calibers.... You can see that below Mach 0.85 you want a small base (50% is optimum).... However, above Mach 1, the optimum base diameter is much larger, as shown by the lines on the right.... The key is a shallow taper, which means longer boattails can (and should) have a smaller base....



Between Mach 0.85 and Mach 1, we need more detail, as shown in the graph below....



You will notice that for short boattails, as is typical of many of my designs that are made to fit magazines or work in existing twists, at 1000 fps the optimum base is 70% of the caliber.... Even that small a base is hard to obtain, Accurate Molds, for example, can't even approach making something that small.... As the boattail gets longer, for a given velocity, the optimum boattail diameter decreases.... However, when we look at what would be optimum for the .308 BBT Whiteout, we are pretty close.... We will be at the optimum at about 1070 fps.... but operating near the minimum drag for velocities well below that....

Bob

[CHUCKSTER]

Wow...

[KnifeMaker]

After reading for years that ---

1.  Boat Tail bullets only work at high velocities, I now see it de-bunked. ABOUT TIME !!! Bravo Bob!

2. That there is NO hydro static shock below 1500-1700 fps. I've blown that myth out of the water with my own testing. So-----

I'm testing a new bullet (for me) from NOE that I milled the length down by decking the mold. It has cut exactly thru the center of the radius of the half round lube grove, effectively making it a rebated short base boattail.

The bullet also has a very long ogive and a tiny meplat. It is ultra good at bucking the wind. This week, I will be doing side by side chrony test, and wind shooting with  like weight bullet, from a mold from the same maker, with a flat base, and a wide meplat. So far, it (the Long ogive/rebated BT) is vastly superior. 

As far as flat base bullets shooting better than GC bullets in our Air Guns, I, Tofazfou, Unrepentedsinner and a number of others, have found "Exactlyt the opposite!"

So far, it has been speculated by many here and on other forums, that it may very well be the case that the riffling marks are not being presented to the muzzle blast, making the bullet more stable, with the Gas check design. I can tell you this, I have tested several bullets, from otherwise identical molds, (same design, maker, weight) and the GC has so far, proven more accurate. By quite a bit!

I have posted many targets over on Talon AG over the years. A Lot of them are much tighter that the naysayer here has been brag'n about. LOL!!!

 Mr.Bob, once again, Thank You Sir, for what you do! It is greatly appreciated by a LOT of us!!!

 

Knife

[Driller]

Interesting.
I thought this would lead into.
Reversable slugs, ie : where the cone becomes the tail.
If any one then made the nose conical.
My limited understanding as the boatail gets longer, (as in the Reversable slug) it evens out the BC curve along the mac numbers till obviously hits the mach 1 and beyond.
+ Makes the tail drag and skin drag not on the charts till after the mach 1 and beyond.
Rebated tale makes it act in a similar way.
(even the curve)