HankL. A properly chambered Ackely Improved rifle should be able to headspace on the parent case. That is, a standard .280 Rem. should safely fire in your chamber. From your post, I gather you loaded up new brass to fireform into the improved case. My question is this. Did you resize the brass or run the brass through your resizing die, prior to loading? If so, your die may be improperly adjusted for your rifle, thus creating a headspace problem.
Take a paper clip and straighten it out. Form a small hook at one end so as to make it look like the letter "L", with the hook about 1/4 inch long. Run the hook down into you fired cases and see if you can feel a groove near the bottom of the case, say in the general area of where your case cracked.
If there are grooves in that area, either your rifle has been headspaced improperly, or if you resized them prior to loading them, your die is adjusted wrong. If it turns out that it is the dies, E-mail me, and I'll tell you how to adjust them so this won't happen. A quick way to see if it's the rifle is to shoot acouple of factory rounds in the gun. Then test them with the little paper clip hook. If there is a groove with the factory ammo, them you'll have to have the barrel set back and rechambered.
Hope this helps.