I can't speak for actual complete rifles, as I've never purchased/owned one in that regards. I can say based in my experiences in building rifles that there is most definitely an obvious difference in quality/fit amongst the various brands.
BCM is like that hot chick that you see walking down the street. She is fine as toads hair, would be perfect to take home to mom, and she knows how to cook. However, she aint no Sara Underwood doing yoga.
Here is an idea for your piston 300blk: 1. buy 5.56 ak 2. get definitive arms to do the pmag well mod 3. contract someone to make ak barrel in 300blk (7.62x35) and install 4. hope it doesn't blow due to several other factors that I'm most likely overlooking 5. ???? 6. profit