As the United States seemed quicker on the uptake than China on converting to microfusion power, the US would be able to eventually achieve some form of victory over China.
So is there any speculation on whether or not the Berlin Brigade were kept at full strength, or would they have been drawn down as more personnel were needed for the war in China?
Provided that STANAG was still in effect, then it's more than likely that 5.56 would still be in circulation. However, at least if I recall correctly, and I'm entirely disregarding Bethesda's Fallouts, that at least the US Army had gone to directed energy weapons for all personnel down range...
I could see the "Sergeant York" (https://en.wikipedia.org/wiki/M247_Sergeant_York) coming into full production (flaws and all), as the Fallout Universe tends towards higher defense budgets and more military contractor influence than even our timeline.
I wonder if any of the following made it into the Fallout Universe US Inventory:
https://en.wikipedia.org/wiki/M8_Armored_Gun_System
https://en.wikipedia.org/wiki/FMC_XR311
https://en.wikipedia.org/wiki/Future_Combat_Systems_Manned_Ground_Vehicles#Mounted_combat_system
Some version of the Abrams seems plausible, though perhaps as fuel shortages increased and the Fallout-Verse Abrams were withdrawn due to logistical issues I could fully see a variation of the M60 being re-activated rather than being a front-line MBT.
The most likely scenario is that personnel assigned power armor would be employing the "Davy Crockett" and other battlefield nuclear weapons against Chinese Armor.
However, cavalry in the modern period has either taken on a reconnaissance role, or it has basically become armor in all but name. Power Armor, is more like "Mounted Rifles" whereby personnel ride into battle, but then fight as infantry. I'm not sure where the shock tactics are coming in though?
Well from at least what was mentioned in Van Buren the soldier that provides an escort to the Vault, informs you he's with a Mechanized Cavalry Regiment, which means they're probably acting primarily in a reconnaissance role, with a secondary role as either anti-armor or some form of "dragoon"...
As I recall, Power Armor was supposed to be a frame to carry heavier ordnance but I'm not sure that it would be able to go muzzle to muzzle with a Chinese T-55?