I'm working up an accuracy load for my .284 Win and my 5 shot groups included a couple of real tight 4 shot clusters with a single flyer. I wasn't looking at each piece of brass after each shot (dumb) but when I picked them up I had a couple split necks (caused by old brass - these loads were nowhere near maximum).

I'll repeat these loads but I wonder if split necks usually cause a flyer.

I have no idea but I can tell you to anneal your brass.

I don't make a habit of it, but I've purposely loaded and fired cases with split necks. Seem to work just as well.

I have also pondered this question. On two or three occasions over the years when shooting on the range I have noticed a split neck, from using old multi-loaded brass. If there was any accuracy loss it wasn't exceptional. I always reload to be under maximum pressures but looking for best accuracy.

If I shoot some older Kynoch factory ammo I have for my 404J I usually get a couple of split neck cases out of every 5 or so rounds shot, but I see no difference in accuracy. Provided the case neck was not split before firing, the bullet will have moved into the leade of the barrel before the neck expands and splits so should have no bearing on accuracy.

If it has started to split before it's shot, it'll have runout you can measure with a ruler. Otherwise, I don't think it would be a huge issue.