Thanks (both) for the info. I noticed the brass shown in the photo was noticeably bulged out above the web area. I never have brass fail like that while fireforming 100's of 6mm AI & .257 R AI cases but have had no experience with the .22 Hornet other than to fire a few rounds out of some .22 H single shot survival rifle during training 57 years ago. PPU makes .22H brass. Hornady .17 H Hornet seems to be all sold out.
Lots of people make 17 H brass from 22 H. PPU is a good choice, anneal after forming and before first firing. The biggest issue picking brass is some 22 H is too small in the body, just above the Rim. Rahter than type it all out, read this. More info than you ever wanted on the subject. Forming 17 HH from 22 H.
The 22 Hornet case is not a loading friendly case. They seem to be to small in front of the rim and will split like the cases 243xb shows in his post. In 22 Hornet cases you have to be very careful inside chamfering the necks. I have had bullets to catch on the case mouth and make a BooBoo in the case, Wrinkle folds the neck and that junks the case.
Converted my .22 Hornet to .17 Hornady Hornet and had a similar problem while reforming my .22 Hornet brass
except my reformed brass separated just above the web. I eventually gave up and threw it all away.
Made many 19 Calhoons out of 22 Hornet with both Remington and Winchester brass. Like L. Sherm said, annealing is a must with either. I split necks on about a third on a batch that I got lazy on and didn't anneal. Lesson learned....