Late in the Twilight war, I think main gun ammo will be so rare that anywhere a welder can mount a pintle mount, they will mount a pintle mount. I mean, realistically, on a NATO MBT, you might see one primary mount with a NATO .50 or 7.62 NATO weapon system and an adhoc 'field welded expedient mount ' over a loader hatch and on APCs, air guard positions. In many cases, late in the war, those will have PACT GPMG mount.

https://i.imgflip.com/5zng9l.jpg