This is a picture of an AOW shotgun,
it transfers with NFA registration for $5 tax. To qualify for AOW registration it mus have never had a buttstock on it.
If you took a std. 870 shotgun and configured it exactly to appear the same, it would transfer on NFA $200 tax.
The AOW can cross state lines willy nilly as long as it is legal in the final destination, the short shotgun must get ATF approval to cross state lines.