Split necks on .17HMR is a common thing no matter the gun or ammo, do a little google search on it, or do some reading over on rimfire central. The shelf life of .17HMR ammo is low compared to other ammo don't quote me but think I saw 5-6 years, and storage environment a bit more important. The case is primed before it is swedged down and therefore cannot be annealed so is prone to stress crack over time, watch for date of manufacture when you buy, and only stock what you will shoot in a couple or so years.
Also a warning to all, when shooting be very cautious of a misfire a guy I know had some real bad ammo once CCI in this case, the necks had split and then I suspect damp storage he had a few duds in some boxes. Well long story short, when they went click and you would eject it the lead would stay in the chamber, he was at the range with his kid and the first bad one went click, the kid ejected it saw it had no lead thought she just forgot to cycle it and loaded another and bang.
At the time they didn't even realize what happened just knew the mag plate was bent after the loud bang and the gun now had a lead stuck in it, brought it to me to fix, I got the lead out only one in there and fixed the mag plate, took it to the range with his ammo and test fired it, worked fine so sighted it in then had a misfire went bolt up then back down, click again, ejected and lead wasn't there it was stuck in the chamber, the casing still full of powder and good primer hits. I looked at the rest of the unfired ammo and found avg 11 split necks per box of 50, in 4 boxes, mystery of what happened to the gun when him and his kid were at the range solved and my guess is as told above.
