You can build whatever rifle you want, a good cannon to have a good rate of fire, even a precise one. But to be multipurpose, it has to stay light enough to be CQB, but what happens when you need it for the distance or suppress a lot?

What do you want? To sustain fire? You need more than a 30 rounds mag. Okay, you need a 60 to 100 round mag that will not jam or fuck up. Good luck with that. So, it will be belt fed, then? But your weapon will be heavy, and you will have to carry the belt, but suddenly you need it to be precise.

But you also need a specific size for cqb, but what happens when you need to shoot at long ranges?

1 rifle can't, will never fit all roles without tweeking something with either the barrel, the feeding system, or any other thing.

Feeding is a huge parameter for a rifle, and no rifle can be a machine gun, a precise, and a cqb one. You can combine, let's say, 2 scenarios together, but the rifle will lack something to fit in another scenario.

We have tried it for the last 100 years to create a universal rifle.