Re: 20 Inch AR Accuracy
Accuracy has nothing to do with barrel length, but it does change the velocity at which the bullet is travelling.
Not all barrels will be the same, but 5" at 100 yards is pretty bad.
I would say you should try different ammo as ammo and barrel combo can vary from rifle to rifle how it will perform.
Take as much of the variables out as you can.
1) Make sure the rings/optics are torqued properly.
2) Get another shooter if you are unsure of your marksmanship to see if they have the same results.
3) Have a good front/rear rest to make sure that you are not pulling the shots.
4) Test out various ammo as not all ammo are created equal. I have always used Hornady V/A max as my standard when it comes to .223/5.56.
Typically, I'd say a 1:7 twist would stabilize a 55gr projectile, but it wouldn't surprise me if it's keyholing. Have you seen any signs of keyholing at 100 yards?