Any AR15 variant would really serve you well. Just pick one that suits you in whatever caliber you decide and stick with it. Nothing wrong with good ol 5.56/223 for anything under 500m.
The A2 birdcage is obsolete
Please explain. They work just fine with all the other AR's in use by hundreds of 3-gunners, myself included. sure there are better options, but in no way are they "obsolete".