AFAIK, M110 has a calibre of 203mm.

So, while the NATO might have standardised on 155mm, nope: I do not think 203mm is going to be discarded any time soon. If for no other…

AFAIK, M110 has a calibre of 203mm. Perhaps 'rare' nowadays, but still heavily in service in Israel, just for example.

So, while the NATO might have standardised on 155mm, nope: I do not think 203mm is going to be discarded any time soon. If for no other reason then to have something to counter Russian 203mm.

And yes, D-20s are in service in Ukraine.

Re. how are they used: shoot at Russian artillery from longer range than it can fire back.