Because availability of a variety of .308 projectiles and an almost infinite supply of parent brass.
I think it bears mention that the Russian was born out of the same fire as the Kurtz, the first urban combat theaters.
The need for something with greater killing power than the subgun but less recoil, and carry weight as the battle rifle.
The 556 likewise was born of a previous conflict, Korea. Where the American troops were greatly outnumbered and need to have a large capacity and low carry weight was born.
In each case armies built weapons based on the prior failing and desire to prevent reoccurring issues. And each time missing the mark on predicting future conflicts needs.
The BO is very much an American x39/ Kurtz. Albeit 50+ years late.
The need to project heavy bullets at closer range in urban environments due to the failure of the 556.
I don't thing it's the absolute answer, personally. But its a step in the right direction IF...BIG IF...we are to maintain the Stoner platform.
If we abandon the rifle and build something as innovative as it was, then we can invest in a different concept.
Who knows what that may be.
Perhaps a x47 with a mono-metal...I can't say.
But for now, as an urban defensive round the BO trumps the 556.
As a practical hunting round for medium game at practical distances the BO trumps!PS the 556.
As an interior assault or MOUT round it trumps the 556.
And if needed, yes , as a heavy and suppressed cartridge, it trumps the 556.
In fact, if I was riding shotgun outside the green zone, hunting deer, protecting my home and neighborhood post disaster, assaulting an urban position or serving a HR warrant I'd choose the BO.
