I did the "20 TCM" thing. Custom made reamer and dies. One pass with a Wildcat Forming die got me to 20 caliber with no issues. I liked that process better than full length die sizing. Factory brass with the proper head stamp is tough as nails. Good looking finished product.
So what you end up with is a shortened .223 case necked down to 20. You just reinvented the wheel but you "just had to do it" to see what you would get.
Well, somebody had to do it.
And on top of all that work, not one shot fired for accuracy. Popped a couple to see about forming but nothing (yet) from the bench. I have high hopes.
Did the 17 VHA thing. Didn't meet my expectations but I really didn't give it that much of a chance. Once I saw the TCM brass, I was hooked and the 17 VHA got moved WAY to the back burner. Just collecting dust. Got brass and dies but not much interest now. Working with the TCM is so much easier, I can't go back now.
And depending on what action you use, you may have feeding issues. The 17 VHA is a little devil. You need to see what the guys ended up using. The TCM may be another feeding problem. Short round may not feed. My Savage action has issues feeding the TCM. You have to start the round into the chamber, not just drop it into the action and close the bolt. That may be a good enough reason for me to abandon that project. Time will tell.