Although there were a variety of AT guns used by all countries throughout the war, I like that for now all factions have their standard issue AT gun. They’re pretty general purpose, can deal with most threats, good all round AT guns. But the Soviets are using the GERMAN PAK 40.
Give the Soviets their own proper AT gun. It’s nice for immersion
ZiS-2
But why not mount that on a plane? 
Better mobility and all
I’d go with the 76.2mm F-22, it’s very comparable to the PaK 40 (Panzerjager Marders would be equipped with either the 7.5cm PaK 40 or the 76.2mm F-22):
(Yes I know it’s Wikipedia, but it gives a good general overview of the gun).
Not really, the 76.2mm F-22 was modified by the German so it can use a much powerful shell.
The original Soviet one is not comparable to Pak 40.
Totally overlooked that part in the wikipedia page (duh), so that’s on me. 57mm is likely the best candidate then, though the HE would be far less powerful compared to the PaK 40.
I was also surprised when I found out that my low Br Soviet engineers can build a Pak 40. Shooting Pz. II tanks with it is overkill, makes me feel bad. Meanwhile, my low Br US engineers have a pea shooter for an AT gun.
The Pak 40 became Germany’s standard AT gun in 1942, the M5 AT gun became the Allies’ standard AT gun in 1943.
Theoretically they can change Germany’s AT gun to the PaK 38 that was their standard at the start of the war so there is relative parity between all factions.
Since Darkflow mentioned there will be high-tier AT guns implemented, I’d honestly spread out 3 different AT guns for BR 1 (~45mm, ~70mm pen), 2-3 (~57-75mm, ~120mm pen), and 4-5 (~76-105mm, ~200+mm pen). So much disparity at top tier because the biggest US can really go is 3-Inch gun M5, or 17 Pdr if they want to use Brit stuff, but of course they could use HVAP.
Every country had a variety of AT guns and it would be great to have them represented in game.
I have thought of various ways for them to be implemented. Of coarse there’s the BR method. I’ve also thought that maybe different Squad types have different AT guns. For example, your regular Engineer squad can build let’s say a 57mm while an AT squad can build a heavier 75mm
On a separate note I still believe the AT squad should have Flak Cannons as their unique Engineer building
early war campaigns (Moscow/Tunisia) did have reasonable and less powerful AT guns pre-merge.
USSR used the 45mm 53K and Axis Pak 38 …
After the merge they sadly didn’t manage to code the squads’ buildable AT guns to what matchup they’re in. So they did a lazy thing: removed all unique and less powerful options (for GER and USSR)
and coded Pak 40 to all squads (engineer squad/AT squad and premiums).
That’s why we see Pak 40 all over the board regardless of the BR in the battle you participate.
Extremely stupid and disappointing that they haven’t come up with a solution to this.
Should’ve been fixed by now.
The problem is Pak 38 is not that good against br3 tank. The old matchmaking will let br 1~2 meet br3, the Pak 40 at least let them to have better way to counter br 3 tank. But I think it should be changed now since we have the soft rule.