I knew I had posted some on the 120 mm BAT and have just found these threads (didn't turn up in advanced search):
http://www.bocn.co.uk/vbforum/threads/70768-120-mm-BAT-case
And this:
http://www.bocn.co.uk/vbforum/threads/72419-120-mm-BAT-recoilless-gun
Sorry, I have no diagrams of the shell. Other info, from 1979, as follows:
User: Infantry.
Equipment: Battalion Anti-Tank (BAT) - obsolete
MOBAT (Modified BAT)
WOMBAT (Weapon of magnesium BAT)
CONBAT (Converted BAT)
My notes say that all the above had the same gun but different cartridges. The BAT and MOBAT had a LMG attached, for ranging purposes, while WOMBAT and CONBAT were fitted with a .50 inch spotting rifle, firing the M48A1 / M48A2 rounds. When the main armament was fired, 80% of the propellant gases escaped through the rear venturi - the danger area was 275 m at an angle of 533 mils.
Ammo in service (QF Fixed): Round 120 mm BAT HESH
Round 120 mm Prac SH
Drill round 120 mm BAT
Round 120 mm BAT HESH L19A3 - HES nose pad
RDX/WX6 91/9 + 0.5% zinc stearate
Fuze Percussion Base Medium L19
Tracer Shell No 30
NQ/S propellant in brass cartridge case
Igniter RCL L1A5
Round 120 mm BAT HESH L43A1 - differs from L19A3 round by having a steel cartridge case & Igniter
RCL L3A1
Round 120 mm BAT HESH L44A1 - differs from L19A3 round only by having Igniter RCL L17A1
Round 120 mm BAT SH Prac L18A5 - used the same empty projectile as the HESH and Tracer No 30
Filled HES and plugged
Brass case with Igniter RCL L1A5
Round 120 mm BAT SH Prac L45A1 - used Igniter RCL L17A1, otherwise no difference from the L18A5
Drill Round 120 mm BAT L6A1 - hardwood body, brass base, brass cap, secured together with a steel
bolt.
By late 1992 the L43A1 HESH round and L18A5 Prac SH round were no longer in use. The M48A1 and M48A2 .50 inch spotter tracer rounds were still listed.