If I have a weapon registered as a DD, does the ammo gave to be taxed as well? I saw the tank thread with 120mm gun and wondered about the rounds for it. Do they also carry a tax stamp? This whole hobby is so confusing and that sucks. Of course .gov is the main problem. This stuff might come in handy in the future.

You might hate this answer, but the most honest answer to your question is "It depends."

There are a lot of variables at play.  Some DD ammo is taxed, some is not.  Some requires a federal explosives license and an approved and regularly inspected storage magazine, some does not.  Some can be obtained as a private person in America, while a lot is only made overseas and only able to be imported for government sales.  


That said, I am a huge proponent of DD ownership, and have been spending a lot of time and money trying to help the DD community.  I hope you jump onboard.

I'll give it a shot:

-all reloading propellant (including the stuff for sale at your local gun store) is technically a banned explosive unless you obtain an explosives license from ATF.

- an EXCEPTION to that rule covers reloading powder for non-DD firearms up to and including 50 BMG. That exception is what allows you to buy and store small amounts at home (the amounts and storage rules can vary by location).

-grey areas: 20mm pull down power can be used for both 50 BMG (non DD) and 20mm (which is a DD).  So it's sold for use with 50 BMG.  Even slower surpluses powder could probably be used legitimately with guns like the 50 BMG, so their sale is usually ignored.  

OTOH, it would be hard to argue that you plan to use 16" battleship cannon powder in your 50 BMG; that powder would probably require a explosives license and approved magazine.

Loaded fixed ammo: generally exempt if the projectile is a solid practice projectile.

Feel free to correct me if I'm wrong here.

Just speaking to large bore destructive devices (cannons, grenade launchers, mortars) the gun needs a stamp.  The ammo depends, however due to cost most of the ammo people shoot does not.

Definition of DD excerpt:


For terms of the law,  missile=projectile.

Cannon shells with inert projectiles or with only a small amount of explosives do not require a DD.
Anything with more than 1/4 once charge requires a $200 tax stamp per round. This cutoff is why .50cal Mk211 and certain 20mm HEI, although hard to find is not a destructive device and are legal to own without a tax stamp.

Besides NFA tax stamps, there are also explosive licensing, and magazine issues to take into consideration if firing anything besides inert projectiles.

I'm thinking about boring out some of my 20mm TP projectiles and seating 50bmg API/APIT/Spotter projectiles.

It's apparently trivial to cut off the front of a tracer projectile and bore out a hole in the back of a TP projectile to make a tracer round too.

As long as the projectile has less that mm 1/4 oz of explosives it's fine. All solid core rounds are good so you can shoot a massive artillery or tank gun as long as it's a solid steel round.