We reported on this gun when Miller Precision first introduced it on July 4th,2012. More recently they displayed the MPA300 at the SHOT Show, 2014. At Media Day they had several rifles there ready for writers to test.
Not too surprising, the writers all wanted to shoot the 300 Winchester in favor of the other models in 308 Winchester and 5.56 Nato.
The MPA300 has the distinction of being the only 300 Winchester Semi-auto on the market that is truly on the AR platform. While the upper, lower, magazine and bolt carrier are specific to the MPA, the rest of the parts are all AR-10 interchangeable. So you can dress your gun with any of the various features that are already on the market for the AR-10.
Media day turned out to be a real endurance test for the MPA300. When Brandon Miller saw that everyone wanted to shoot the 300 Winchester almost exclusively, he decided to see how long the gun would run without a cleaning. It went about 2700 rounds before the first malfunction. A quick inspection and cleaning had the gun back at the line and by the time the smoke cleared the gun had fired over 4000 rounds without a second failure.
Actions are machined from billet aluminum. These guys are selling quality, not big-box store discount products. Finishes for the Miller Precision guns are about as varied as you can ask for. Among other things they offer hydrographic coatings, so the choices are almost limitless. These guns are impressive.
Brandon Miller with his MPA300, Media Day 2014
GunsmithTalk 
Any comments on observed accuracy of the MPA300 would be appreciated.
Thanks!
If you watch the video attached they talk about accuracy. I shot the gun at Media day, SHOT Show 2014. I worked flawlessly and everyone I watched was able to ding a steel plate at 500 Meters or so, that was the longest target available on that part of the range.
That is a seriously sexy gun and it sounds like it’s pretty reliable being able to put 2700 rounds through w/o a cleaning and only have 1 misfire. I actually like the fact that it has multiple finishes available and can use most AR-10 parts.
I’m glad to see that they are focusing on high quality instead of mass producing junk.