Quoted:
Sorry if these questions have been asked many times before.

I own a bushy M4, so I want to build something completely different.

I want to build a 20" varmint style rifle and my main goal is accuracy.
(I will be using an existing stag lower and add a quality FCG).

To start with, I have 4 dumb questions:

1. Who makes a good (not insanely expensive)  upper assembly with a
   target barrel, ff handguard, etc.>>>>>>>>>>>>>>>>>>>>>>Bushmaster
2. Does a flash hider degrade accuracy?>>>>>>>>>>>>>>>>>yes at extreme ranges in a controlled environment it will, outside in the real world you wouldn't be able to tell.

3. Do I need 1:7 twist for super heavy bullets?>>>>>>>>>>>>>There are charts for each type, its best to read up on this.  this is one of those things you MUST know, not just get someone to vomit out an answer.

4.  Is rigidity of the flat top receiver a concern when trying to
   build a sub moa rifel?>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>Yes, so don't do something like cut a viewing hole to watch the bolt cycle...ya I have seen that.

Thanks.