8 Driving Bands or 4 Driving Bands?
I have been working close with Cutting Edge Bullets, and my pal Sam Rose on the new BBW #13 Solids. The first prototype 500 gr CEB #13s I ordered in .500 caliber with multi-bands, eight total driving bands. While there were some other issues with these bullets, pressure was one of those. With the normal load of 70/IMR 4198 these bullets were giving extraordinary velocities of 2180 fps, but pressures were slightly high at 67727 PSI!
After learning more about driving bands, bearing surface and pressures from the double rifle tests, and a few others, I ordered a new prototype version. I did not want to sacrifice performance, but wanted the pressure lower, hoping to be at or below 65000 PSI as maximum safe pressure.
While I made some changes that would effect the terminal performance by a better more rounded radius, and a bevel base for ease of loading, the big change was going from the 8 driving bands to the 4 driving bands, reducing the amount of bearing surface significantly.
When the new prototypes arrived I decided to try another test also, seating the bullet out longer, and seating deeper.
I wanted to see how much difference there was between the two seating depths as well.
Seated Long= 59581 PSI
Seated Deep= 60646 PSI
Not really enough difference to worry over. Seated deep, feed and function is improved, so I elect to seat these bullets to the depth you see on the right side.
Wow, the decrease in pressure went from 67727 PSI with the 8 bands to 60646 PSI with the 4 band bullet, over 7000 PSI. This gives this load a very safe margin to work with. Not only that, but performance remained the same, maybe slightly better at 2195 fps.
While this is great, there are other factors to be taken into account. One of these is the fact of whether the bullet has enough gripping surface to remain in the case, without recoil causing it to move. In the case of the 50 B&M it the 3rd round down in the magazine did not move at all. In a magazine rifle, most of the time recoil will beat the bullet into the case, not so in the 50 B&M and this load without a crimp.
In the 500 MDM I put a good crimp below the top band kept the bullet from moving rearward in the magazine.
The new bullet has been approved for production runs, and is official now. Not only did pressure reduce significantly, but terminal performance was incredible, and velocity remained high for both cartridges.
In an additional test with the 425 CEB BBW #13 Copper Bullet, I ran those in the 50 B&M for the test work. Again the comparison between the 8 band bullet and the 4 band bullet, with the same exact load.
The 8 Band Copper 425 with 74/IMR 4198 give me a pressure reading of 60105 PSI. The 4 Band version brought that pressure down to 55603 PSI, a difference of 4500 PSI on average. Enough to make note of.
Since these tests were done we have changed the bands to a new configuration, instead of evenly spaced, we moved the two bands in the middle up to the top band, leaving the bottom band as is. The main reason is to give the loaded cartridge better neck tension, holding the bullet in place better, and giving options on seating and crimping if needed. I do not think that the reposition of the bands will have any pressure effects at all. This will be tested of course, and reported if there is a change in pressure, either up or down substantially, right here in this report.