ATI is among the last manufacturers I would consider. Their ARs don’t have a very good reputation. They are most known for polymer ARs (upper and lower receiver) which are poor quality parts. I think their “Milsport” line, which is aluminum, may be a response to that perception, but looking at the website, they don’t list a complete AR in that line, just a lower receiver for a 9mm pistol caliber carbine. I own one of their polymer uppers (bought for a dedicated .22 project) and it is a very low quality product, even by the standards of polymer AR manufacturers, many of which are junk.
Honestly, there are far better choices. I would start with Palmetto State Armory. Either buy a complete rifle, or a complete upper and a complete lower; they snap together with two pins and assembly will not be a problem. You can easily find something in the same price range as ATI and it will be far higher quality.
Right now, they have a nitride upper with a rail, backup iron sights, and a nickel boron bolt carrier group (a significant upgrade) for $299, and a complete lower for $129. This combination is far better than anything ATI offers.
https://palmettostatearmory.com/psa-ar15-freedom-classic-lower-flat-dark-earth-7779346.html
I am not worried about “concealed” carry—I have several 9mm’s.
I expect the future requirement is going to be for something that can be open carried but is lighter weight and less cumbersome than an assault rifle of which I also have several.
I am looking for a “pistol” that shoots 5.56mm; I have not resolved the 10” vs. 16” barrel issue; I am willing to spend more than bottom dollar to get better gear.
I think the objective is to be able to respond to a shooter threat where I need to be able to hit a target probably closer than 100 yards. I assume some kind of minimum optics would suffice.
I want a sufficient quality barrel that I can practice shooting with it for a while. Is the flash suppressor necessary for both the 10” and 16”?
The CAV-15 (from Cavalry Arms, and later from GWACS Armory) is a definite exception to the 'polymer ARs are junk' point of view. That's because the CAV-15 was designed as a polymer lower receiver assembly, and isn't just a plastic "clone" of an aluminum lower receiver. The integrated buffer tube, stock, and pistol grip actually make it stronger (and in some ways more reliable) than a standard aluminum lower. They're also lighter than comparable aluminum lower assemblies. Unfortunately, the molds have apparently worn out (after ~15 years of production), so no more CAV-15 MKIIs will be made. (My recommendation, for anyone wanting a reliable, light-weight AR15 - find a CAV-15 while you still can! ;^)