I want to build something really short with law Tactical folder. I am thinking 300 black out is my best option but I'm not sure what else is out there.

300 BO would be my thought

Blackout or 9mm

best ballistics is going to go to the 300 blackout or 6.8spc

most affordable to shoot with adequate terminal performance for HD purposes will be 5.56 and 9mm

i'd be willing to bet the 22tcm would make a good contender but that would more of a custom job vs something off the shelf. ronald williams comes to mind, and using ppsh 43 mags would be my first try.

If you're not gonna suppress it, then just go with 556 and get a muzzle device that directs the blast or at least something more than an A2.

Keep in mind any blast deflector type muzzle device will add at least another couple inches to the length though.

My own experiences on  those  blast deflectors (longer than an A2 by quite a bit):

--Noveske flaming pig

--AAC three prong FH with AAC QD blast deflector (very spendy since you need both parts)

--Silencerco three prong FH with Silencerco's QD blast deflector (same story on being spendy as the AAC brand, two parts)

--KAK or another brand that's lighter in weight and less money will likely do 99% of what you need.

Anyway you slice it, a decent blast deflector will add another 2" or so to your length and add weight, but shooting 556 out of an 8" barrel with an A2 sucks for everyone, particularly the shooter.

If you can live without a rifle round, I don't think going with a pistol caliber is it worth it, considering Glockz and others can pack 15 to 30 rounds and be hand-sized.

A nice option IMO is the Fn FiveSeven handgun, with their hotter ammo, also available from Fn.  The  FiveSeven, for its size, weight, mag capacity, and power is hard to beat and would be a first choice before compromising from an 8"  barrel in a rifle caliber to any length barrel in a pistol caliber.