I'm not saying Europe is UN-available, but I could also easily see scenarios like "Europe, desperately trying to concentrate its domestic arms production on building up European militaries, instead pays a third party to make the Commonwealth weapons."
Shit gets complicated. And when we are getting so far down into the weeds that we're talking about individual gun calibers or vehicle models on the basis that we "know" what countries will be supplying the weapons we haven't even begun making efforts to import...
We're making way too much soup out of way too few oysters.
While I would normally agree, the reason I suggested these calibers in the first place is because Europe is currently undergoing a military buildup, but not on an active war footing.
This means that their current limit to arms manufacture is not "how many can we physically produce," but instead "how many can we pay for without raising taxes so much that our economy is crippled." In this specific situation, foreign nations buying arms from you has a net effect of growing your military industrial complex, which is extremely important in the lead up to a major war.
Effectively, from the majority of Europe's perspective selling arms to us is a win on every possible level.
1. They can afford to grow their military industrial complex more than they could have otherwise (not much, but every bit helps).
2. They weaken one of Russia's puppets, either accelerating the return of the United States to the world stage or forcing Russia to divert resources from dealing with them towards propping up Victoria.
3. They what may well be the future United States will owe them one, making it easier for a future American government to sell the idea of intervention in Europe to its people.
4. They get to help kill fascists, which the soon to be Prime Minister of Germany (and likely many other heads of state as well) will view as a massive benefit.
5. They get combat data on how their weapons perform against Russian weapons. It may be some of their less advanced weapons against some of Russia's least advanced weapons, but some of the design philosophy will be constant between technological generations.