A couple of things to try. Trim Hornet brass to the 17AI min length. Remove seating stem from your seating die lube case well and run into seating die, replenish lube and try running into full length sizing die. If that works your done.But if cases don't collapse when run into seating die but continue to collapse when run into sizing die do this. Take the case that has been run through the seating die and try chambering in your rifle it should chamber ok and if it does then prime the case add about 2 to 3 gr of Bullseye or similar fast burning powder, carefully fill case with uncooked cream of wheat to within about 1/4 inch of top of neck make a small wad of tp or paper towel and top off with that to help hold every thing together. carefully load into rifle with muzzle pointed up to help prevent spilling and fire, some adjustment to the powder charge may be needed to get fully formed cases. Seems like a lot of work and a waste of primers and powder but may be the best route to go to keep from ruining brass. Good luck.
