The very best defensive drills involve shooting at multiple targets, ...
I suggest placing three targets so that they appear to be a minimum of two feet apart, from shoulder to shoulder, with one target at seven yards from your firing line, one at 10, and one at 15. (Every time you do this exercise, change the order of the targets' distances/positions.)
Start facing the center target, draw, and fire a shot at the nearest, then one at the mid-range, and finally one at the

hest away. Then, without stopping, immediately go back and fire one more shot at each target.
Or fire two on each, and then, without stopping, go back and hit each target's head with one shot.
You are interested in smoothness, not speed. Speed will come with time and practice.
You are also interested in the quality of your hits. Your body shots should all hit center-of-mass, and when there are two of them in the target, they should be close together. Your head shots should be properly centered in the target's head.
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