Split Necks when Fire Forming >243 AI

[gasph]

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.

 

[Rev.]

Did you anneal the brass first?

 

[gasph]

I did not anneal and do not know how to or have the equipment to do it but it sounds like I may need to learn or else buy some of the premium brass that this is already done to it ?

 

[BradArnett]

I've have quite a few AI'd rifles and have only ran into split necks while forming one time. It was when I was using once fired brass to form for one of my 223AI's. You say that you do have the .oo4" crush fit so it's not that. (personally) I like to put the bullet in the lands when forming cases to just to make sure that I have positive headspace. I also like to form with max to near max loads for the parent chambering. It could simply be a couple pieces of brass that was harder than the rest also. I'd just form them all and not worry about losing a couple cases, but that's just me.

 

[Trickymissfit]

I never have had the split neck syndome with my 6/250AI during fireform, but I also build my case out of .243 brass instead of 22-250 brass. And this leads me to thinking back when several friends of mine got on the .243AI and 22-250AI kick. All were plagued with split necks as well. The guys that shot the .243AI ended up using 6mm brass or 6.5x55 brass, and making the necks as long as they could fit in the chamber (one guy reworked an older .243 barrel with a neck that was about .075" longer than the one in the good chamber. This pretty much ended the problem for them. I anneal my necks prior to fireforming, and seldom loose a case. Looking back, I think that annealing the neck and shoulder area might have solved the problem for everybody. Interestingly these same guys shooting factory loaded rounds in their rifles did not have much of a split neck issue. Still thinking back thru the years I've only had split necks with cases that have short necks. Why I don't really know for sure.
gary

 

[gasph]

Update: I shot one more of the 41gr loads with the 70 gr speers and the neck split again which was no surprise. I pulled all the bullets and dumped the power and started over again. I reduced the load to 39.1 gr which is a minimum load and they are working fine and blowing the shoulder out like they should. I think this solves the problem.

I also ordered 100 Lapua cases and I will see how they do. Thanks for the replies.

 

[Skyking]

I just fireformed 100 Lapua cases with 40g 4831sc with Amax 105. Never had a problem. Then finally load was 47.5g of H1000 with Berger 105vld 3295fps. I anneal every time after fire forming.

Here is the 5 shot group with the Amax's

 

[gasph]

That is an impresssive group and it looks like you are measurning outside to outside. I have to shoot light weight bullets due to the 14 TW I have. What twist are you using?