A couple or three years back I went up to Ponteland with an old mate, a medal-winning shooter whilst in the army and still a regular on the ranges.Shades of the US Army in the 1980s, after noticing the difficulties their soldiery were having in making bovine posteriors and ukuleles coincide even in sideshows like Grenada.
Cue a massive "Advanced Combat Rifle" project with various technological marvels, intended to improve the hit probability of the fighting man by 100%. Duplex bullets! Super-rapid three-round bursts! Hypervelocity flechettes! Optical sights! (back when the British were strange aberrations for putting glass on a rifle)
And yet, and yet, most of the various technobabble fell by the wayside compared to the consistent, significant improvement achieved even with the iron-sighted M16A2 when shooters - of a range of backgrounds, militarisation and assumed skill - were simply given a weapon, plenty of ammunition, targets, and feedback on where their rounds were going. Even without coaching, the improvement from "practice and feedback" swamped the improvements offered by the assorted sci-fi projects.
The problem is that we're now in a death spiral where the standard of shooting, and the skills involved, are so low that "because it looks ally" or "because it gets more rounds down" trump actual effects achieved, and as a result myth and prejudice dominate (for instance, rating the LMG as better than the LSW for suppressive fire, simply because it turns more rounds into brass faster).
I'll repost some commentary and linked info from @Gravelbelly a couple of years back on this very thread-
I refer Sir to these posts by the esteemed @dogmonkey, fresh back from operational employment of the then-new L86A2 on TELIC 1 (fifteen years ago...)"LSW A2 - excellent suppressive to 800m. And by suppressive I do not mean loads of splash landing randomly around your fire trench and making a noise. I mean suppressive by way of bloke next to you pops his head up and gets a smoking third eye. You going to stick your head up now? And with the LSW you can do this with considerably less weight than a GPMG.""Prior to crossing, we had the ITDU out to 'train the trainer' on the new kit. They were at pains to ensure that we didn't ditch the LSW. Just as well we didn't. The LSWs were excellent weapons which allowed engagements at much longer ranges. The minimis were used to good effect from FSp locs within their shorter range when a large volume of fire was required. Both A2 variants worked exceptionally well and were totally reliable. The desert cleaning regime was not onerous, and when doused in oil there were virtually no stoppages, and none were reported in 3 weeks of fighting which had any serious repercussions. The troops' confidence in the weapon system is now justifiably high. "
But again, misuse the weapon thoroughly enough and it gets a bad reputation, which becomes self-reinforcing (because you've given it to the crows, and not trained its use, and it's just a longer, more awkward rifle...) except for the few times someone manages to (briefly) break the cycle.
I hadn't shot full-bore in decades. A bit of .22 pest-control on farmland in various places around the world, but that was it.
It came back rather quickly, with bulls starting to happen regularly.
My mate's comment?
The ACR competition you mention is one of several that have failed over the decades to produce enough of an improvement over the AR series. As you say, there are reasons why and they're not technological.