The Tube, The Twist
Assuming no glaring errors were committed in the construction of the rifle, if your AR-15 doesn't shoot well, it's the barrel.
And there ain't no such thing as a "match barrel." No universally accepted standards exist for it. Different makers set their own standards to grade their own work, but that's not good enough. "Match barrel" is a judgement made by a shooter, not a manufacturer. No one can stop barrelmakers from claiming to have them, though. The best way to judge a barrel early on is by its price. Good barrels cost money, but it's the best money you'll ever spend on your rifle.
So, what do you do before you get disgusted with a barrel that isn't performing as it should? Before pulling it, have it recrowned. Fixing a bad crown can have a miraculous effect. Another option would be running a FinalFinish kit through the barrel. This is a polishing/uniforming process that can have wondrous results. Personally, I wouldn't cryo-treat the barrel or spend more than the first two options cost. Put the money into a new barrel instead.
Twist matters most when big bullets are in your plans. Bullet length, not weight, determines the twist needed for stability. Longer bullets need a faster twist. The most popular twist rate is 1:9. That's not enough. The best all-around twist is 1:8. That will let you shoot anything up to and including Sierra 80-grain MatchKings. It will also work fine with a 77-grain MatchKing or other similar heavier bullet designed to be loaded to magazine box length.
Long .224 bullets like the VLD designs from Berger and the 80-grain Sierra and Nosler or 75-grain Hornady A-Max are not intended to be loaded to a magazine box OAL. They are for use in the portions of NRA High Power, such as 600-yard "slow fire," wherein rounds are single-fed.