One thing about the 300BO that caught my attention, it's possible to chamber and fire a 300BO round in a 223/5.56 chamber. There are pics of kabooms out there. That's not a deal-breaker for me, but good to be aware of, and always double check what's what.
This is true, BUT - it's not that hard to just avoid using 300 Blk ammo that will chamber in a 5.56. That didn't really exist commercially until a few years ago, and only happened to handloaders when using shorter bullets. The original intent, using bullets profiled like the 125 NBT, was that the ammo would not chamber in a 5.56 except by repeated attempts to hammer the bolt closed. If you use a bullet like the 110 V-max and seat it deep enough, the round will chamber in 5.56 because the bullet doesn't hit the chamber shoulder first.
Personally I thought it was a huge mistake when Nosler started selling their Varmageddon load that would chamber in 5.56 barrels. I don't know if they were the first to do so commercially, but they were the first I knew about.
