With what I have learned over the past few days about bullet design and the fact the boattails will in fact work to reduce drag at subsonic velocities.... I decided to develop a design for a .25 cal boattail airgun bullet.... There were several criteria to be met....

1. It had to fit a standard airgun bore with 0.250" groove diameter....
2. It had to be stable in a 14" twist barrel at 1000 fps and less....
3. It had to fit in a stock .25 cal MRod magazine....
4. It had to have a Meplat of about 1/2 the caliber....
5. It had to be convertable to a HollowPoint design....

In addition to the above, I wanted the highest subsonic Ballistics Coefficient that I could achieve.... I wanted to use a Tangent Ogive design with the longest nose radius that I could.... and yet maintain the longest driving band length for bore alignment (conflicting requirements).... I did quite a bit of research on Rebated (stepped) Boattails, and found the Corbin website very helpful in explaining why they made sense, and how much of a step worked.... The advantages are a cleaner release from the crown, less blowby, and on average 15% smaller groups than a conventional Boattail (where the tail meets the body at an angle).... but there is a slight increase in drag, primarily when supersonic.... One additional advantage I came up with is that since the step is not at the base of the bullet, any slight base imperfection from the casting process (eg. the sprue cutter) won't affect the area that releases from the muzzle crown, ie the base is further back and less likely to cause accuracy problems.... The part that releases from the muzzle crown is the back point of the rear driving band....

I played around with a variety of dimensions, changing the proportions and seeing what affect they had on the BC and Stability.... I consulted with Erik at HollowPoint Mold Services in regards to the HP cavity dimensions.... and I eventually settled on the following design....



The subsonic Drag Coefficient is half that of a flat base bullet of the same dimensions, giving a BC of 0.20 for the HP and 0.22 for the RF design.... The Solid (RF) weighs 51 gr. (in pure lead), and the HollowPoint should be about 46 gr.... The Stability Factor is just over 1.5 at 1000 fps, and is 2.2 at 500 fps in a 14" Twist.... If launched at 90 FPE, it would have about 80 FPE remaining at 100 yards, assuming the BC lives up to expectations....

I showed the design to Sean Pero, he pondered it for a while, and then offered to make a Cherry cutter and make me a single cavity mold to find out how it works.... I know Sean is a busy guy, but with Winter coming, maybe, just maybe, he'll find the time and I'll be able to test the theories....

Bob