Before Project Babylon, Gerald Bull had some rather more practical pursuits. One of these was the Extended Range, Full Bore shell. Let’s take a look.
Here’s a standard 155mm HE shell from the time period, the M107. It’s quite fat, and it has to be. Those brass rings are the driving bands, and they engage the rifling of the gun to be sure the shell spins. The driving band also drives the design of the rest of the shell and the placement of its center of gravity. This design is not very good from an aerodynamic perspective. Bull reasoned that if he could only get some other way for the shell to engage the rifling and spin, he could improve the shell.
And this is Gerald Bull’s Extended Range, Full Bore (EFRB) shell, also in 155mm. We can see that it’s much narrower and that it lacks a driving band. There are also some curious nubs about the midsection. These nubs engage the rifling, filling the role of the driving band and ensuring the shell is spin-stabilized. Once we change that part of the structure, we can narrow the shell body and improve its aerodynamic qualities. The end result was much improved range, though at the cost of accuracy. More range could be attained with base-bleed or rocket-assist packages for the shell base. Let’s get to some data.
|Shell||Weight||Muzzle Velocity||Range||Probable Range Error||Probable Error in Line|
|M107||95 lbs.||2,210 fps||11.1 mi.||194 ft.||39 ft.|
|EFRB||100.4 lbs.||2,940 fps||18.6 mi.||620 ft.||138 ft.|
Figures in the table are from a 45 caliber-length barrel and a 23 L chamber. Probable Error is the figure such that 50% of shells fired will fall within that figure of the target. “In Line” is perpendicular to the range (i.e. we might think of this as ‘horizontal’ or ‘in bearing’).
We can see that there’s a considerable increase in range, but also in dispersion. The accuracy issues meant that it was never procured by NATO in bulk, but the range meant that many other countries would seek out the shells. The South Africans found them very effective in border conflicts with the Angolans, giving the South Africans an option to strike back at the BM-21s1 of the Angolans.
As always, what do we think? I really like EFRB. It gives artillery some options to reach out further. The accuracy loss is bothersome, but they needn’t be the only shells bought. And they at least give the opportunity for a longer range shot.
Verdict: Approved for Procurement by the Borgundy Army Ordnance Procurement Board
- Maximum range of the standard BM-21 rockets are 12 miles. ↩