...these things are treated first as functional engineering challenges by teams of developers rather than ergonomic packages to be carried by combatants humans...
Yup, and not just for gun. The rare project I am allowed to get on early enough to do the "ergonomic package" work much better. Engineers can work within the constraints, or we discuss how to step outside them, and do tradeoffs. (Yes, done some hardware systems; love to do a weapon program if anyone wants to chat about it!)
Since I do this practice area for a living, and write on it and so on: there's not too much value in having engineers with experience in the end user space. Humans are bad at analyzing, don't have all experiences, etc. etc. What you get is someone solving what they remember being their problems, from back in the day, which often are not generally true, are mis-remembered, and are not the current problem. Professional design (UX, HF, etc) can do this right with observation and heuristics.
It is annoying as hell no one on the PM side seems to be driving much from what the Army knows perfectly well as human factors baseline info and requirements. Lots of good data from ACR, and programs since then, even if no time/money to get newer info. But instead we get these Nerf guns with zero obvious human consideration. I'd hope the evaluation includes that but if these are the downselected guns, the competition must have been full of poisoned spikes.