Does a faster burning powder make a big difference in loudness & flash?
In this case, yes, it was noticeably less.
& what about ballistics? I ruled out a short barreled .223 a while because it seemed like the obnoxiousness was too much to deal with. Especially coupled with weak ballistics.
Depends on what you mean by weak ballistics. External ballistics will not be as good, obviously, due to lower velocities, but you would not be using this powder for long range shooting. Terminal ballistics depends on the bullet selection, and there are plenty of bullets available that were designed for lower velocities, but you might have to look for projectiles intended for for other cartridges, such as 22 Hornet. I don’t know how effective these would be for self defense loads, though.
Just as a comparison, the velocities I recorded were as follows:
Xtreme 55 FMJ, min charge of 19.0gn H4198, CCI #41 primer.
8.5” AR pistol - 2196fps, cycled fully and locked open.
16” AR carbine - 2490fps. Did not fully cycle.
At a charge of 20.5gn in the carbine, I got 2779fps. No data for the pistol.
For a self defense application, I think a different caliber, such as 9mm, would make more sense in a short barrel AR when you consider noise, flash, and terminal ballistics.