3catcircus
Adventurer
It's covering the techniques used by shooters in close quarters battle (e.g., clearing a house or otherwise fighting in tight quarters). Basically, in GURPS, you take a penalty to attack with a gun if you are moving while shooting; the CQB technique lets you spend character points to reduce that penalty.
FWIW, OP, the way to do it is to figure the number out once, and write it down when you write down the gun stats. So if you are using an Barret REC7 (to pick the last gun on the table on p. 63), have Guns (Rifle)-15, and have invested 3 pts in CQB, you might write down, "Barret REC7 (skill 15, CQB-14, 5d+1 pi, Acc 4, Rng 780/3300, ROF 12, shots 30+1, Rcl 2, very reliable)", or whatever.
Or just write down that you've bought off X pts of penalty (where X is 1-4, depending on how many points you spent on CQB) -- that's what CQB lets you do.
The "use the lower of your modified CQB or unmodified shooting skill" just means that you can't have Guns-14, CQB-18, use a gun with a -2 penalty, and fire at (CQB-18 minus 2 equals...) skill 16.
This.
I've encountered similar situations when playing Twilight:2013 - the solution is to have a weapons card with all applicable stuff on it for each weapon you carry. This even works well in 3.x/PF where you could have variables for different feats, etc.