Ok, I know a 20mm requires a tax stamp as it is a DD, a mag fed Anzio is on my bucket list
When it comes to ammo, solid core practice and AP rounds do not require a tax stamp.

But What about 20mm high explosive rounds?
I found somewhere selling what appear to be live 20mm Semi-Armor Piercing Hight Explosivre (SAPHE) rounds.
Would these require a tax stamp for each round or are the rounds small enough that they contain less than the 1/4 oz of explosive Making them not restricted by the ATF? Similar to the variety of 30 and 50 cal HE and incindiary rounds.

And if that's the case at what size round do the restrictions take effect? 25mm, 30mm, 37mm, 40mm?

Thanks!

Edit: never mind, seller contacted me and it appears they are inert. Likely more than 1/4oz of HE anyways.

I believe it has more than 0.25oz of HE compound. Mk 211 has only 1 or 2g IIRC so that's why it's legal.

Interesting, I wonder how They are able to sell them than. I emailed them but have not heard back yet.

The PGU-28/B has 10 grams per wikipedia.  However, the cite link is broken.  It's possible that the 10 grams is both the HE and incendiary compound.   The actual HE content may be less than 7 grams.

I suppose I could always make/load my own HE rounds with just 7 grams in each. Apparently I know where to buy the inert empty rounds

20mm is right at the threshold.  Some are over 1/4 oz and DDs, some are under 1/4 oz.

The issue is finding someone willing to sell some...

This is one of two issues. The other is that 20x102mm must ship by freight as it isn't small arms ammunition, and the prices to freight anything considered explosive (other than 1.4S - which includes 20x102mm with inert projectiles - or 1.4G) can be high. I'm not entirely certain whether 20x102mm would be 1.4B or 1.4D if it was loaded in such a way to have under 7g of HE, but in either case it would be a headache to ship.