Not mil spec, low production quality. I had one of there rifles when I first got into ARs. I tried to upgrade the system only to find no parts other than the ones that came with the gun would fit on it. I sold it off and educated myself that was in 1998. I have spent alot of money and time with the weapons system in the time since then. If you want an AR upper get a good one.
For example I have a Bravo Company USA 14.5" upper in one of my rifles its just a plane jane copy of the M4 I carried in the Army. It has well over 3k rounds through it and I can still hit a man sized target at 500M with iron sights. That upper cost me around 600$ new. It is 100% mil spec and maybe even a little better.
I suggest calling Grant at G&R tactical and having him build you an upper or just buy a pre built model. The options are varied to say the least. If you tell me what you want to do with it I can make some suggestions? I know you said MOA and 400 yards and for the most part thats cake for any AR.
I'm a 1-7 twist guy all the way I like heavy bullets 77g. With most hobby guns your going to get a 1-9 twist which is ok but not optimal. If I was building a LR ar in 5.56 it would have a 1-7 twist. Here is a link to the chart EDUCATION is key here.