This is my understanding. These things are gas-driven. The 300 BO has a very limited amount of gas due to the very small powder charges being used. So a longer gas system allows the gas to de-pressurize as it stretches out over a longer length of barrel.
For the 5.56, smaller bore, more gas produced by larger powder charge, more forgiving on a longer system. But dwell time can become critical. Too little barrel ahead of the port can allow the gas to escape too quickly creating an "under-gassed" situation.
There's more to it than that I'm sure, but that's my take on the basics of it.
That's a pretty good explanation.
Here are a few factors that require a shorter gas system:
- faster powder
- larger bore
- smaller case capacity/powder charge
The 300 Blk has all of those.
The OP statement that "longer is better" is oversimplified and incorrect, even for 5.56 - in barrels 16" or shorter, you can certainly go too long with the gas system. The whole gun is a system that needs to be balanced right, you can't just push one variable to the extreme.