When looking at drill rounds, you have to remember that they are supposed to be loaded into and out of guns any, many times, where a live round only gets loaded once or twice.
So they make the things VERY solid, so that they will take the battering that the gun gives them...... and the battering that the crew gives them...... and not be wrecked in order that they can be used and abused again and again by new crews being trained.
And because the men coming in are not used to handling ammunition of this type, they make the things VISIBLY DIFFERENT (that super-deep extractor groove is an instance: live rounds did not have it) from a Service round, as well as MARKING them clearly. Clearly, they want NO accidents.
And this also has a bonus in that, when you are in the sights of a crippled Heinkel which is looking for a spot to lay its eggs, nobody gets all confused and shoves a clip of DRILL ROUNDS into the feedway! That well could be....... embarrassing, what say?
But that's what you have, friend.
Likely more than a few guys on this board would like to have one of those.
Enjoy!
.
.