Their ballistic and gun technology is as advanced compared to the modern day as the modern day compared to WW2.
The funny thing is, gun technology has plateaued in recent decades. We're at a point of refining designs that are known to work. Here's a few speculative things that never quite off the ground which might be in your world.
This is all centered around increasing hit probability and increasing the amount of ammunition one can carry.
RPM (Rounds per minute), FPS (Feet per second), Energy (Dispersed to
target), Ammunition Capacity, Ability to mass produce
Two of these things are not like the others in terms of "advanced": energy and RPM.
The energy of your standard military rifle round peaked in WW2 and then dropped when everybody switched to intermediate rounds. Full rifle rounds like .30-06 or 7.62x54 deliver 3000+ J and are designed to still be effective at 2000 meters, but were found to be unnecessarily heavy and difficult to control in burst or automatic fire. Most shooting was found to be done at 300 meters or less, so everyone switched to lighter and more controllable intermediate cartridges like 5.56 NATO and 5.45x39.
Similarly RPM has generally dropped to allow for more controllable fire. Many battle rifles dropped their fully automatic settings and instead are single-shot or burst fire.
While we're certainly capable of producing monstrous rounds with high rates of fire, your average soldier probably won't be carrying them unless there's a good reason, like an arms race with advanced armor.
...armour that is nigh on indestructible even under heavy fire from armour piercing rounds out of a Barrett 50 BMG
Assuming this is the goal, 50 BMG packs quite a punch: 18,000 J. No current body armor will stop it, the scale ends at Type IV designed to stop armor piercing .30-06 with 4000 J. 50 BMG will go through 20mm of steel at normal combat ranges. Furthermore, designing body armor to stop one round is one thing. Designing body armor to stop a hail of rounds is another thing.
The surface area of your average adult human male is about $2 m^2$. At 20 mm thick that's 40 L of steel. At about 8kg/L that's about 320 kg of steel. If these folks are 10 times stronger, that's the equivalent of 32 kg. Heavy, but very good for full body armor. And that's just the upper end using steel. You can use whatever handwavium you like to further reduce the weight.
Small armor plates which overlap ingeniously, connected with smart, flexible handwavium, will provide flexibility yet never leave a gap.
As discussed above, if your average soldier has to hump around and fire 50 BMG their effectiveness and hit rate will drop dramatically. 50 BMG is already enormous with a tremendous kick. Soldiers won't be running around firing 50 BMG. It has to be either a specialty weapon, such as an anti-material sniper, or vehicle mounted. Carrying and firing anything much larger or hotter gets ridiculous.
Instead of ratcheting up the calibers and energy they'll probably switch to 20mm HEAT rounds and man-portable anti-tank weapons such as Javelin but shrunk down. While they'll be able to penetrate the armor, this will still greatly reduce the effectiveness of the average soldier as now they get a few slow shots whereas before they could carry hundreds of rounds. The armor will still have done its job.