Okay...so an 18" is the shortest barrel you can have.
That means with a conventional pump action, you are good to go in terms of a folder or pistol grip stock. 100% legal as far as our firearms laws go, given that your OAL will be over 26". An 18" 870 with a top folder (or even a Knoxx SpecOps folder) would fit the bill, but you'd need a tall backpack. You're probably looking about 30" total.
As far as actual take-down models...I don't have any first-hand knowledge.
Legal as far as our laws go... but they won't let me into the US with that gun. No folder, just basic shoulder stocks allowed. Not even pistol grips.


















































