I look at being in the "bubble" as being in a state of concentration that nothing around you, you will notice.. Just as in like when I shoot prone matches or national match type rifle matches.. the person next to me.. When it is time to focus, I am aware of the fact that I am not the only one on the line.. but those others, they are lost in my mist of concetration. A person could be shooting a 338 Lapua Mag next to me, I will not notice that they are there till it is all over. My bubble is: me, the target and my scorer. My focus zone, is me and the target. Notice that I have not mentioned the firearm.. that is because, in part, my way of thinking is that you must become part of the weapon that You using. In essence " Be the Bullet".
I do a lot of "imaging", going through everything in my mind. Position, sight alignment, calling the shot. See everything in your mind. Over and over again.
Always remembering the basics, building upon them.. when I come into a problem, I start over from the foundation.