From CoH-Stats
Accuracy Increment
Please note that this is not your standard Accuracy Multiplier, I will try to explain it with the following example.
MG42 has an Accuracy of 0.125 at long range.
Its Accuracy Increment Modifier is 1.12.
At long distances
When the MG42 is shooting at 1 man, the accuracy is 0.125 (12.5%)
When the MG42 is shooting at 2 men, the accuracy is 0.125*1.12=0.14 (14%)
When the MG42 is shooting at 3 men, the accuracy is 0.125*1.12*1.12=~0.16 (16%)
When the MG42 is shooting at 4 men, the accuracy is 0.125*1.12*1.12*1.12=~0.18 (18%)
When the MG42 is shooting at 5 men, the accuracy is 0.125*1.12*1.12*1.12*1.12=~0.20 (20%)
When the MG42 is shooting at 6 men, the accuracy is 0.125*1.12*1.12*1.12*1.12*1.12=~0.22 (22%)
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Notice its not 1,2,3... squads, but 1,2,3... men.
To take advantage of the extra accuracy bonuses, all of the men must be standing within the search radius.
If the target is outside of the search radius, only the default accuracy apply. Search radius has not been extensively tested.
So what this means is that the same MG42 at long distances, kills one member of a 6 men riflemen squad (6/6) faster than killing a single rifleman on his own (1/6).
Bike and Jeep has a Accuracy Increment Multiplier of 0.975, which translates to reduced accuracy when there are a lot of units, so less damage.
Accuracy Increment Chart