Obersoldaten are a good example. They use focus fire with the MG34 resulting in 1 model taking all the damage whereas a Kübelwagen distributes the damage among the units in the scatter circle.
I find the whole "re-roll" idea quite absurd.
From my understanding there are no "re-rolls" - the target chosen for a "hit check" is chosen among the entities within the scatter radius (including the "primary target", which is the center of that circle)
I was under the impression that no focus fire does the re-roll once on misses, not that it will randomly aim at any unit in the squad first. Possibly test with kubel vs soviet sniper team? If it does re-rolls then the model it targets should usually die first, if it doesn't either model has equal chance to die first.
My experience in game tells me the targeted model dies first a majority of the time. Look at this video that was in trending recently, the targeted sniper squad takes 2x the damage that the other squad takes. With re-rolls working the targeted entity should always take more hits than the others. Example with 30% accuracy:
3 outcomes
Model 1: 30% hit 70% miss
Model 2: 30% * 70% = 21% hit 79% miss
Nothing: 70% * 70% = 49% both miss
That's kinda logical since you don't want burst weapons to annihilate squad members one by one.
The Obersoldaten MG34 has it set on true, explaining why they are so good in killing models.
No focus fire is just a straight up buff to the weapon, literally an extra accuracy roll. Obers without focus fire would be even more deadly.