High Power and 3Gun bear almost no resemblance to one another. Those sports have about as much in common as Tai Chi and American Kempo.
That said, there are some commonalities between the equipment of competitors in either game. Even though a 3Gun rifle and High Power rifle look nothing alike, there is a similarity. Almost universally, competitors in either discipline shoot rifles that have a float tube installed on them.
There’s a reason for this, of course. If a rifle doesn’t have a float tube on it, any pressure exerted on the barrel will cause it to bend slightly, but still enough to change the point of impact of the rounds. This pressure can come from anything, such as being slung up tightly, or leaning against a barricade, or shooting from an improvised position that causes pressure to be put on the fore end/barrel.
The thing that sucks about this is that it can cause you to miss.
The solution is to install a float tube. What the tube does, is remove any contact between the hand guards and the barrel. This frees you up to exert as much pressure on the gun as you want, helping you to get a steady hold, while at the same time making sure none is exerted on the barrel itself.
If you’ve bought a plain-jane factory-AR rifle, a good argument could be made that the first thing to do to improve accuracy is the installation of a decent trigger. The second would be the installation of a float tube.