Ballistic Accuracy Computer vs 122mm

I tried to match the official firing tables of the 122mm guns using the BR-471B shell.  The first matches the DDR (Deutsche Democratic Republic a.k.a. East Germany) 1960 table for the D-25T tank cannon with a muzzle velocity of 781 m/s.  The ballistics of the G7 drag table match better than other drag tables.  However, the drag had to be reduced at longer ranges.  When a function (noted by K#) that reduces the drag is applied a close match to the official range table can be produced.  Note that this is just a small change in the ballistic coefficient but it does make a difference in impact velocity of about 10 m/s at 3000 meters.

The second problem was to match the Soviet firing table produced by the D-25 gun.  This had a muzzle velocity of 795 m/s.  In order to get the D7 drag function to match the table a smaller reduction in the drag has to be applied at a higher rate than the D-25T gun. This alteration is noted by the k/,  While the guns muzzle velocities differ by only 14 m/s the change in the drag function is different by a factor of nearly 5.4 times.

DDR firing table for 122mm D-25T APBC BR-471B.

Comparing Ballistic Accuracy Program to DDR firing table after drag adjustment.

Soviet firing table for D-25 APBC BR-471B.

Comparing Ballistic Accuracy Program to Soviet firing table after drag adjustment.

Figure to the right:
The selection of the drag function at these ranges only matters in terms of the difference in the muzzle velocity (V0) and terminal velocity (Vt). (Green lines) For the game that usually is at ranges of 3000m or less. At these ranges the drag factor to feet per second is almost a sloped straight line.  The slope being the main difference. Though the G1 curve looks a bit convex here while the G7 slightly concave.

When the velocity of the projectile gets closer to the speed of sound indicated by the blue line the function

gets complicated. There is a large divergence in curve pattern around 1500 f/s.  This is around the range of 6000m.
In a naval game where ranges are 15,000 yards and more a different drag function might better model the overall ballistics.  For my naval game I use the G1 drag model for all guns. As the ballistic model has to take into account the top of the ballistic arc where the velocity gets into subsonic range.

Below:  The Russians incorrectly calculated the ballistics of the 122mm D-25 gun in their 1944 firing table.  The red circles the height and time of flight to 1000m above LOS.  Green circles show the same for 2000m.  It shows that the 1944 numbers for the BR-471 are close to the BR-471B data in the post WWII firing tables.

Top data is from FT-F-122-2 US document of the captured 1944 Firing tables.   Lower data is from post WWII firing tables.   Examples:   122mm BR-471   BR-471B    or O-471