The Spigot was 29mm on the outside. The Spigot, though fairly solid, had a spring cocked firing pin running through it. The bomb had a hollow tail tube in which was a propellant charge and primer cap. The spigot, which was fixed to the traversing unit, fitted inside that bomb tube. When fired the tube (and bomb) moved up and off the spigot and continued onwards. Think of it like a rifle barrel with a cartridge, except the barrel gets projected but the cartridge stays. The firing principal is the same as what was used in the later PIAT.
According to the Wiki article the AT bomb was 20lb with an anti-personnel bomb of 14lb (9.01 and 6.35 kg respectively). Those 2 types had ranges of a bit over 100m and about 500m respectively (say 3 and 12 hexes).
Turning the BB into ASL terms is a bit of a guess. Due to its low velocity, the best comparison is with MTR rounds. The Soviet 107mm fired a 9.1 Kg bomb, the 82mm fired a 3.05 Kg bomb. Due to it's lower muzzle velocity compared to medium mortars (range only 100m or 500m), it wouldn't be outrageous to suggest that the shell casing would proportionally lighter than a MTR, IE higher HE content.
So for the
14lb APers round: Range 12, 100 mm HE equivalency. Definitely needs to be bigger than 82mm as twice the weight.
20lb AT round: Range 3, 120/150 mm HE equivalency. While the AT round at 9 Kg should match a 107mm, its likely higher HE content and reputation of its blast being quite dangerous to its crew even at 100m suggests at least 120mm, possibly even150mm. From what I gathered the AT round was not a HEAT type, just a great big blob of standard HE.
So what of the rest of its stats? I would rate it as INF, T, [0] RoF, B11, M12, R1/X6, No Tow, DMable to 5 pp DM. Alternative AT, T, [0] RoF, B10, M12, R2/X6, No Tow, DMable to 5pp. In either case it can fire only on the VTT or ITT, but not on the ATT. Leaves no residual. HE only, range 0-3, 120mm/150mm or 0-12, 100mm. It is not a MTR. Due to its muzzle loading, I am tempted with an NoIF, but given the low weight of it's round, I bet a crew could get off another round at a charging panzer.
I must warn that all the above is speculation, but should not be too far from the ASL truth. The BB is basically an earlier and oversized PIAT.