There will always be those (writers and others) who insist that this or that caliber are inadequate. I tend to rank them .22, .25, .32, .380, .38, 9mm, all the rest. None of them hold a candle to a good rifle or 12 gauge. I say carry the biggest one you can. Sometimes that means a small caliber.
I suppose the NAA revolvers are smallest in .22 or .22 mag. Once you go above that it seems you may as well jump to some kind of .32...the .25s aren't any smaller guns.
If you can conceal a little bigger, then a .380 or .38 makes sense. A short barreled .38 or .380 may not be as good as a long barrelled .38, but it no doubt beats any .22, .32 or .25.
There is plenty of data out there. A .380 +P out of a NAA Guardian in my mind is a serious round.