>>63826765
>i would of assumed the 5.56 was tracer and you used it to sight the round.
Nope, the NIVA is select-fire and mean to function as an assault rifle, which is why it's pretty retarded.
>there was some other gun that did that
Lots of artillery pieces use spotting rifles for sighting like that. Pictured has one.
https://guns.fandom.com/wiki/NIVA_XM1970
The tl;dr is that it was 100% intended to combine the 5.56mm assault rifle and the recoilless 45mm rifle, and it did do that, functionally, but the problem is that this is not something which you want.
Soldiers would be mostly using 5.56mm, so they have this needlessly heavy 45mm recoilless launcher on their rifle which they aren't using much, and they are carrying 45mm shells for that thing instead of more 5.56mm ammo.
Not stated in that source, but which I believe probably is worth thinking about, is that 45mm is pretty small for a recoilless rifle, range and payload is definitely not going to be on the level of the 84mm Carl Gustaf recoilless rifle, so one would really wonder what the point would then be.
A much more practical solution to giving a soldier some additional explosive firepower would simply be a 40x46mm High/Low grenade launcher attachment, which may not have the same kind of long range or flat trajectory, but which is going to be MUCH less heavy and bulky, both for the weapon and the ammunition. Optionally, the soldier could carry something like a lightweight and single use 66mm M72 LAW, or a somewhat heavier but still pretty light and handy AT4 (aka M136) 84mm.
