Kinda the whole point to "improving" a standard case is to burn more powder, by going to a barrel that short you're negating the benefit of being able to put more powder in the case. I know many will argue but ackley improving the 223 case, with pressures being equal, in a 26in barrel is usually only about 100 fps gain over a standard 223. Some will argue they're getting 150 or even 200+ fps more out of their 223ai's and I'm sure they are but I can promise you it's not at equal pressure/s.

I have a 223ai with a 22 inch barrel and for the most part, I'm only getting about 50 fps more from it than I am some of my other 223's, in fact I have a standard 223 with a 26 inch barrel that shoots the same bullet a little bit faster. I can load my 223ai hotter and make it shoot faster than the others but it will be the result of higher pressure and I could easily do the same with my 26 in. 223 as well.

If a 223ai is what you want, by all means have one built, I love mine and shoot it as much as any gun I own, just be careful listening to people talk about fps numbers and how much faster a 223ai shoots over a standard 223 because, again pressures being equal, it's really not that much of a gain and with a barrel as short as you're talking about, it would be minimal. I hunt coyotes with my fireforming loads because they shoot so well but they shoot slower, because the larger ai chamber lowers pressure, than the exact same loads do in my standard 223's. When I think about it it's actually quite ironic, my first round of loading for my 223ai shoot slower than they do in my standard 223's then the second round of loading only shoots about 50-60 fps faster than my standard 223's. As much as I like my 223ai to do all over again with only using a 22 in. barrel I'm pretty sure I'd save the expense of another set of dies and go straight 223 and if I was going shorter than 20, there is no way I'd go AI.