OK now, the new Cooper 57M is coming around nicely, all five shots touching but noticed from last box of 17gr. VMax, case necks cracked all the way through neck and shoulder on several cases? Caused by??? r in s.

This has been discussed over on Rimfire Central a number of times.
It's quite common.
Mainly due to the work hardening of the brass during manufature apparently and is nothing to worry about, there's nothing wrong with your rifle.

Headspace off rim.

Nature of the beast. I have two rifles and arevvlver in 17hmr. They all will have a split case from time to time. For me though the hornady cases are the only ones that split.

It doesn't matter what "brand" the .17 HMR ammo is as it is all virtually the same -- and manufactured by CCI. Particularly in firearms in which the chambers are a bit oversized (as in the TCs), you'll get the cracks/splits with any brand you use.

It doesn't seem to affect accuracy, however.

Ironically, I've had 3 Marlin rifles in .17 HMR and have never seen a split. But these relatively inexpensive rifles seem to have some of the most consistently well-done chambers out there -- and in casting them, all 3 were on the minimum side of SAAMI spec.

+1 for the split cases in TCs.