I'm still on my first play, just taking my time, moving from one town doing quests before moving on to another and exploring. Since the shotguns I found early in the game (single shotgun, caravan shotgun, lever shotgun) are all 20 gauge, I have been converting all of my 20 gauge shells into slugs. When came across 12 gauge shells, or got low on 20 gauge shells I would buy 12 gauge shells and break them down and made 20 gauge slugs too. Now I have acquired a 12 gauge hunting shotgun. As soon as I came to the next reloading bench I was going to break down some 20 gauge slugs and re-fill all of those 12 gauge hulls I have, but there is no option to break down the 20 gauge slugs. This seems silly not to be able to break down custom ammo that I will no longer use.

Yes, I know there are (several) mods for this, but it seems like something that was overlooked and should be in the game.

There is a fair bit of ammo that you can't break down. None of the AP ammo, basically, the only ones that you CAN break down, are the standard, run of the mill, 'ball' ammo. This strikes me as a bit of an oversight. I am sure a mod will fix it though for those on PC.

What I do is break down everything into its parts and only make as much new ammo as I know im gona use up and leave the rest as weightless bits. Then in shops and trareling merchants I always ..buy.. the spent cases and hulls they sell as well as powder and whatnot.

Check your Repair skill, sometimes ammo recipes require certain REP scores to break and make ammo. God knows .45-70 and .50 BMG need like 80-100 REP, but they're SOOO worth it.
Don't know why 20 ga would give you trouble, that's one of the first ammo types I broke down, I hoarded handloaded 12 ga. buck, magnum, coin shot, and slugs early on before I got the Winchester M-12, the Dinner Bell, and the Riot Shottie.