All the stats in the game files show no reduced scatter from vet 2 to vet 3. And veterancy is directly applied to the gun, not the "unit". There is a trigger for when the weapon crew dies the gun loses the stats it gets from vet, but a lot of units don't have that because ~Relic~. For instance the OKW Pak43's gun retains vet bonus's even after it's decrewed and you can stack new vet on top of it.
So when the gun is swapped out for the vet 3 version the vet 2 bonus applied to the default gun is not reapplied (or accounted for in the gun stats)?
And here I thought removing the auto fire bonus from the pack vet 2 wasn't symptomatic of a larger problem >.>.