I don't get the need for a piston gun (much less an adjustable piston system). If you're using it for long range work at the local F&G a DI gun will do everything you want and the idiosyncracies of DI guns and accuracy are well sign posted. You'll end up shooting one load, probably a heavy bullet VMax or VLD loaded close to the max length the magazine will allow. The nice thing about DI guns with longer barrels is that they're fairly pressure agnostic. Also don't confuse heavy barrels with accurate barrels, there are situations where a barrel can be too heavy for the upper housing which basically destroys the upper.
No.
The twist of the barrel will determine its purpose and what GRAIN ( weight ) of the bullet it is plan to be use with.
Ex. A faster twist like the 1/7 will be better for heavier bullets. The 1/12 is quite the opposite. Made for lighter bullets.
The heavier the bullet is the more torque it will need from the barrel twist to stabilize it. Therefore a faster twist.
Mainly the difference between a long versus a short barrel you will get is the bullet velocities. Longer barrel will give you higher velocities with the same exact ammunition. Higher velocity will have a flatter trajectory and will go further. More precise? Up to certain distances yes.
Building your first AR from scratch you will end up with a "Franken AR" that will be marginal at best. keeping it simple by waiting for the right assembly you are looking for to pop up in EE, than change parts out later on as you know what you like and don't like.
Some do, some don't. Marginal stability can also be lost if conditions (altitude, temperature etc) change.
My 20" 1/9 stabilizes 77gr SMKs just fine.



























