Lessons from the Hero of Indianapolis

I am seeing accounts that his accuracy was in part bracing against a solid surface.

This point cannot be too strongly made.  We are taught isosceles shooting stance, sometimes Weaver, because the emphasis is on sudden confrontation with a peer or near-peer attacker.  He has a pistol and so do you and short distances (7 to 25 yards).

 But an active shooter scenario may (and often does) involve someone with a substantial advantage in accuracy (rifle or shotgun) and range (20-40 yards).

Some years ago when I was a kitchen table FFL, I sold a couple of Browning Hi-Powers to friends.   Out at the range, I showed them firing Winchester white box FMJs with the frame (not the slide) braced against a wooden beam, I could get 5 to 7 shots out of 10 on the 100 yard gong.  Soon they were burning through ammo that I sold them at an entertaining rate.

I am not sure how practical this will be at an indoor range but it is worth trying at least to find out what brand is most accurate in your gun under those conditions.

Praying that you will never need to use this skill but please be proficient if needed.

The high precision field of view is about 6 degrees.  As we all know, peripheral vision is pretty limited as to information processed and thus brain availability.  Unless a mass shooter is consciously sweeping the area for threats, attacking from 60 degrees off his target area might catch him by surprise.  Worth a try if you can get there steathily.