not exactly. we are getting into NFA (national firearms act), federal laws. in an AR style rifle, anything below 16" is a short barreled rifle and requires a tax stamp. the exception is if it is a "pistol." this would require no stock or an arm brace in place of a stock. also there are some other restrictions. As for your state laws and local laws, that I do not know because I live in a rather free area (for the time being).
Now idk anyone that tries to conceal carry a 300blk pistol. that is not the point of having a 300blk pistol, same as most people wouldn't try and conceal carry a 44 magnum revolver with a 5 in barrel and many other types of pistols. the point of a 300blk pistol is it is short, light, easy to maneuver, and great for home defense or as a truck gun. also it is fun. and one huge reason why long barrels were never a thought in the minds of the developers of the 300blk is because it was made to put a subsonic round in an ar15 with the same capacity and without replacing anything but the barrel. now, you can get supersonic rounds flying at reasonable speeds for a rifle, but subsonic always has to be slower so that it doesn't break the sound barrier. so doesn't matter if the barrel is 26 inches or 6 inches, it still needs to remain under those supersonic speeds, so that's really where your barrel length is not going to matter much.
for most people, 300blk is a round that can be chambered in an ar15 with only changing the barrel and keeping the same mag capacity. it is also a round that can be shot supersonic or subsonic with a can. now if you will always be using supersonic ammo, than I see the benefit of the longer barrel, but if you want it for its subsonic capabilities, in my opinion I see more benefits from a pistol or short barreled rifle setup.
either way, any gun is a thing of beauty to me and I'll support your 100% in what you choose and I look forward to seeing what you build. ?