I am getting around to putting my first postban together and need input. First off, when I store my AR-15's away in my vault I separate the uppers and lowers so they all fit in there. Now if I go off and throw a post ban weapon in with prebans am I putting myself in danger of possible legal problems if say the "authorities" come in and slap one of the preban uppers on a postban lower and say I built a illegal weapon? Also, in putting together my postban I have an extra preban colt barrel and wonder if I cut off the bayonet lug and locktight a muzzle brake on it would that be acceptable? Thanks!
                         

This is a gray area.  Generally the "constructive possession" to create a machine gun rules wouldn't apply, since owning machine gun parts is illegal, but owning uppers with bayonet lugs is not illegal.

As long as you've got a legal way to assemble everything in your safe, I wouldn't sweat it, but that's just my personal opinion and tolerance for risk.

To be absolutely safe, you should leave them assembled (buy a bigger safe), and all in legal configuration, with no spare pre-ban parts, or even get rid of all your post-bans or pre-bans, but I think that'd be excessive.

locktite isn't enough to "cover" the threads, according to ATF - you have to use blind pin and weld, or high temp silver solder to permanently affix a legal muzzle brake to have it be post-ban legal.

What Troy said.

The cases that deal with intent involve readily convertable weapons and illegal configuration with components that don't have legal counterparts on our near each other.