so, basically, the idea of firing ports on an infantry fighting vehicle were a failed concept at inception.

in classic military fashion, it was "planning for the previous war". The idea behind firing ports were that infantry inside the APC could contribute to perimeter security, since armored vehicles tended to have a lot of blind spots.

While they sort-of work as viewports, there is no way to allow a regular infantry weapon to fire out of the ports without compromising the envelope of the inside of the armored vehicle, so dedicated weapons systems were developed.

But, these dedicated weapons systems failed at their intended job because they reduced the available space inside the vehicle, and these were already cramped.

Then, we entered the era of asymmetrical warfare.

No longer were armored vehicles engaging in open fields where enemy infantry could approach from any angle and the firing ports provided wide open sight lines that were arguably critical, instead more and more fighting was being done in tight quarters and especially urban environments.

Unless you were parked in the middle of a crossroads, the firing ports were basically useless.

So, the guns came out, and the firing ports weren't used anymore.

Now, what to do with all these weapons in inventory? they're still valid weapons. These in particular are basically an M4 but with a higher cyclic rate. no front sight because they werent intended to be shoulder fired. there was an idea later to update them with a fixed front sight post in the carry handle, but by that point they had been out of regular use for so long it was deemed not cost effective since you could just throw an optics mount bracket on the carry handle and use that instead.

Some units, especially heavy infantry and cavalry units that had a lot of these, in the early days of what would become OIF/OEF/GWOT, put them in hmmwvs to serve as backup weapons for the turret gunner. And in this capacity, they were great. Small profile, high rate of fire, and used standard stanag magazines so they needed no additional logistics.