I just ran a difference check between the OST mortar and the USF mortar. There are a lot of differences that are to USF mortar's favour. Some bugs exist (which might cripple USF mortar's performance).
The differences between the two mortars are the following:
(+) Vet1: USF mortar receives -20% scatter on all attacks (instead of useless counter-barrage)
This is significant bonus, and is exactly enough to offset the penalties of firing into the fog-of-war.
Since this is an area, you can say that the OST mortar will be ~50% more inaccurate than the USF mortar starting from Vet1.
(+) Barrage: USF mortar fires a lot faster
The two mortars have different wind-up and wind-down times (1.5 for USF vs 2 for OST, for each). What this means, is while the two mortars are barraging, their reload rate will be:
- USF: 3.125 secs per shell
- OST: 4.125 secs per shell
(The USF mortar will also start firing 0.5 secs faster due to wind-up).
(+) Vet3: USF mortar receives scatter bonuses on all attacks (instead of only autofire)
Similar story to Vet1. That USF mortar turbo-barrage will really, really hurt.
(-) Max scatter cap too high for USF mortar barrage weapon
Max scatter for barrage should be 6.4 instead of 8, so that scatter caps at range 80.
Currently this makes no difference because the mortar cannot barrage past range 80, anyway.
Moreover, the Vet1 and Vet3 scatter bonuses to the barrage will be more than enough to cancel this (OST mortar barrage scatter doesn't benefit from Vet). I am only listing this here in case Relic decides to fix USF scatter veterancy.
(-) USF barrage weapon has 20 penetration instead of 35
This one is completely random. Should be 35, like all other mortar weapons.
(-) USF Mortar crew are easier to hit on retreat
OST Crew received accuracy on retreat: -60%
USF Crew received accuracy on retreat: -50%
(that means USF crew is +25% more likely to get hit when retreat)