I am looking for some advice. I loaded some rounds with new Winchester brass in regular .243 Win for fireforming in my .243 AI which is a new build. Two of the 18 rounds I shot split the necks pretty bad, one on both sides. Here are the particulars: 1) 5 rounds with 41.8 gr varget - 58 gr H Vmax - no issues, and they shot 1/2" at 100 yds. 2) 10 rounds with 40.0 gr varget - 70gr speer HP - no issues, they shot about 1". 3) shot 3 rounds with 41.0 gr varget - 70 gr speer HP and they did not shoot as good as the other loads and I noticed two had necks split pretty bad and the shoulder did not blow out clean. I stopped shooting. This is only a mid-level load and is a couple of grains below max by my load books. Primers were CCI LR and WLR I did not track the placement of the different primers in the loads. 4) I have several more loaded at 41.0 and some at 42.0 varget and the 70 gr speers. It looks like I need to pull these and reduce to 40.0 to load with the 70 gr bullets or use them with the 58 grain v-max. 5) I have loaded the fire formed AI cases with all 58 gr bullets and have stepped the charges up to 41.8, 42.5 and 43.5 varget which are all well below max for regular .243 win. I will try shooting these next week. What do you think caused the split necks, is the Win. brass too hard ? My chamber was set .004" shorter than the regular .243 win for the crush fit for the AI. A good gunsmith set this up for me. He thought maybe we should set the chamber back a little more. Since some of the brass did not have issues I am not sure I want to do this. My reloaded FF rounds fit snug in the chamber but eject fine. since I am using the short bullets in a 14 TW barrel I do not think the bullets are or can touch the lands. I am sure most will agree that I should pull the loads or not shoot them in this gun but does anyone have opinions on why I would be getting split necks with less than maximum loads? Anything I should check and does anyone think the barrel should be set back. The barrel was bought used but had little use and is reportedly a shillen varmint # 5 in .243 AI. My gunsmith trued the action and checked the chamber with a go gauge. The cases that did fire form without problems look good. Advice is appreciated.