A friend of mine had a 17hmr , and was forever going about split necks , I thought he was just exaggerating a little however he had an afternoon / early evening with me clearing a few rabbits . He sorted out his brand new box of ammo and presented me with 6 split cases , couldn’t believe it .
A later date we tested them over my LabRadar and found the split cased ammo was around 100 fps ( some upto 150 ish fps ) slower than the normal rounds .
If I remember I will see find the data log and post it up