+1 for JerryM. You can actually use the fact that partial sizing(which is what you are doing) causes the shoulder to rise, to set your full length die correctly: load and fire a couple brass that have been partial sized, then repeat. Often, the second time around, they will chamber hard. Perfect. Now, turn the full length die down carefully, until the sized brass chambers w/o resistance. You have your die set correctly for your gun.
If you want a laugh or two, mark the die position with a marker, then set the die according to the manufacturers instructions..touching the shell holder, then add 1/8-1/4 turn...you will see why so much brass is ruined by oversizing.
Hornady offers a universal caliber neck die. Get one for .25, set it down in small steps until you size the amount of your 257AI neck you want, and you are home. It runs under $20.