Sorry to get into this so late.
About 4 years ago, I worked with Olympic Arms on a prototype rifle based on the .223 necked up to take 6mm bullets. The cartridge is identified as a 6mm X 45mm in the Seirra Rifle Reloading manual.
Since then, I have been used the rifle for both training and duty, (factory ammunition is available from RBCD ammo). The ballistics that I settled on was a 85 gr bullet traveling 2600 fps.
This load, (using the Seirra 85 gr matchking bullet) is capable of minute of angle or better out of the right rifle, (the very accurate Olypic arms rifles do just fine).
I wanted a rifle that was more accurate than a .223, but still as small for patrol rifle duties. I also wanted more punch through heavy cover, and this rifle/cartridge does it all.
I have an ACOG 3.5 X 35 on the flat top. The barrel is an 18 inch barrel with the 16 inch gas system, with a Vortex flash suppressor, (it was a pre-ban). Aluminum handguard, free floated barrel.
The gun can be seen on Jim Cirillo's second video, as I shoot a rapid fire group with 4 of the 5 bullets within 2.5 inches, and the 5th flier just outside 3".
To validate the concept, I attended and H+K precisoin rifle course and used that rifle to qualify with, and did just fine, despite the non-precision reticle in the scope. (it has a 3 moa donut shaped reticle).
Recently, I have been testing another version for better accuracy, it is a post ban receiver, (same barrel), with an Oly compensator, and a Leopould 3.5X 10 tactical atop it. I fired 6 3-shot groups in succession, (no cool off period). First group was half inch, next 4 were about one inch or better, (I didn't measure each exactly), and the final group was again under half inch.
Last weekend, I was discussing this rifle with Chuck Taylor and he was pretty interested in the concept, and Jim Cirillo just loves it. I have been too busy to pursue the concept further, but with a little more time on my hands lately, I am seriously considering pushing the idea further.
, Art