This is a nice diorama that brings back some memories.
A Fire Mission would begin when the Forward Observer (FO later renamed FIST) would call into the Fire Direction Center (FDC) by radio. It would occur something like this... Tango 43 this is Foxtrot 68, Adjust Fire, BMPs with dismounted infantry in the open, Grid 43925744, request Battery 2 Rounds ICM". Then the FDC would manually send a challenge which the FO would then authenticate the message with matching CEOI's as they did not have encrypted digital messaging back then. As the FDC RTO received this call, the chart operator would simultaneously plot the targets grid location and send the range and deflection off the RDP to the person calculating the firing solution. The FDC uses the range and direction along with the ballistic and barametric corrections to calculate the deflection (direction), quadrant (elevation), charge (amount of gun powder to range the target), and if applicable the time (to set on the time fuse).
Most Fire Missions would start as adjust fire missions till the target is ranged. This would be done by the "base piece" which is normally the number 3 or 4 gun in a six gun battery.
For example, the FDC during this whole process would normally call out to the base piece (for adjust fire) "FIRE MISSION, ADJUST FIRE, BASE PIECE ONE ROUND". Remember that for adjust fire missions the round is normally HE/PD until the fire for effect phase. Or it could be as complicated as "FIRE MISSION, BATTERY 2 ROUND(S) CHARGE 7, ICM, DEFLECTION 2438, QUADRANT 496, TIME 32.6", FIRE WHEN READY, REPORT WHEN ROUNDS COMPLETE".
This would allow a portion of the gun crew to cut the powder (remove any necessary bags) from the tied canister charge at a distance from the gun (for safety reasons). In an orchestrated event the gun team would load the desired round with fuze (PD or TIME) set and on the carrier, hook it to the loader/rammer, flip the rammer to the load position, and ram the round into the breach. Only then would they deliver the powder charge. The loader would yell out the charge to confirm such as "charge seven white bag, I see red!". This signifies the powder is loaded correctly and the loader sees the red dot on the powder charge. The breach is closed, then primer is inserted (roughly the size of a 410 shotgun round) and then the firing pin is locked over the primer. At that time is when the lanyard is attached (again for safety reasons). Once all of this is done the gun chief, based on the iniitial commands, either fires or reports to the FDC that they are ready, such as "BASE PIECE READY" (example of an adjust fire mission).
While most missions would be called in first as adjust fires, there were ones that were known surveyed targets, supressive fires, or Final Protective Fires (FPF's) (often danger close), which would normally involve the whole battery up to even a batallion fire mission. This could involve three or more batterys totalling 18 guns. For FPF's or pre planned fires, there wasn't all of this, normally a code word or call sign, and then a self authentication. This is different from the challenge and authentication of a normal fire mission.
Depending on the target the round could then be HE/PD, HE/Time fuze, ICM, Willie Peat, etc. At that time the FDC would normally call the battery to load one round or as the needs and target require 2 or more rounds, and fire when ready, or fire at my command. Firing at the FDC command would either be for effect, or when Danger close to alert the FO of splash for observation and further corrections. Splash would be called to the FO around 4-5 second prior to impact to allow the FO to remain in a protective position and only show himself to see the impact. Most always FPF's would be fire when ready.
The FO normally ends the mission, Tango 43 this is Foxtrot 68, END OF MISSION; BMP's burning and dismounted infantry destroyed, estimate 35 casualities".
Believe it or not all of this would occur with a round going down range in well less than 30 seconds; depending on the proficiency of the FO, FDC, and gun crews.
Yup, I remember when......
Ben