Ballistic Coefficient vs. Accuracy?

[Distant Thunder]

I know that we BPCR shooter work in a velocity range that all other shooting disciplines work hard to avoid as much as possible. The transonic velocity zone, from 1346 fps down to 897 fps, not only requires different bullet shapes than either supersonic or subsonic the requirements apparently change from the upper end to the lower end. Some compromises have to be made.

High power shooters just stay supersonic from the muzzle to the target, we can't do that with BP and lead bullets. Rimfire guys tend to start out at the lower end and transition to subsonic if the range is extended rather than go through the entire zone.

BPTR long-range shooters will mostly start out at the upper end of the transonic zone or somewhat about it and pretty much travel through the entire range. I have seen successful shooter with .45 caliber bullets in the 1250 fps range, but I found that I do much better when I start out near 1300 fps and up to about 1400 fps. I think most successful Creedmoor shooters are starting in the 1300 to 1400 fps range.

The two best bullet shapes for the transonic zone that are in use today are the elliptical and the money. I read recently that spinning these two shapes faster, using a faster twist, is helpful in maintaining stability through the transonic velocity zone. Also having the ogive of the bullet on the longer side can help but can get too long very easily and cause more trouble than it helps. Keeping some nose weight is considered a good thing.

All this gets complicated by how the conditions can push our bullets around, how much they are affected by windshear and quartering headwinds. Our bullets are deflected 3 times more than a .308 with a 155 grain Palma load. Our bullets are deflected a little less once they drop below 900 fps and are fully subsonic, but that's not until they reach 900 or so.

With a bullet starting out above the speed of sound the sonic wave is behind the bullet but quickly catches up to the bullet and as the bullet slows even more the sonic wave moves from the base to the tip of the bullet and eventually out in front of it. The effect of this sonic wave is what everybody else stays away from and we must learn to live with.

I think that if our bullets are designed to cope with the effects of the sonic wave and not so much to just be super slippery with a high B.C. we would be better served, and our scores would improve.

Of course, there is the possibility that the best bullet design might not be the most accurate in the rifle you have. I am not one of those shooters who builds a new rifle every year while chasing the next thing that will improve my scores. I have had some success tweaking the bullet design to get the best possible performance out of an existing rifle and very good accuracy seems to have followed along with that effort.

When I first designed my elliptical bullet, the bullet on the right in the picture above, I wasn't thinking of a high ballistic coefficient design I was just looking for a PPB that would be as accurate as possible in my rifle. The design started with Brent Danielson's prolate design and I modified it to work better in my 18-twist Shiloh .45-90. I doing so I actually gave up a good bit of the B.C. the prolate offers. How much I don't know.

As that bullet is today it has been a very accurate bullet for me and it has held up very well in conditions that other struggled in. I have no idea what the B.C. is and I don't care.

The bullet on the left in that picture is from a mold a friend of mine has and he uses it in his long-range muzzle loader. I only had about 50 of those bullets to work with so I didn't do any load development at all. I simply seated them on top of the same load I use for the elliptical in my .45-70.

I shot these two different times at to different rifle ranges at 1000 yards a couple of years ago. They shot just as accurately as my elliptical and I used the same sight setting as I do with my elliptical +/- one or two moa.

The results might be entirely different in tricky winds or other conditions, but I really couldn't see any difference in how they shot or the sight settings needed.

It's all very interesting what works and what doesn't in this sport and I wish I understood more of the whys and why nots of the transonic zone.