I would bring this to the attention of the folks at Horizon, and see what they say.
They may have a suggestion for you, or may want to take another look at the firearm.
Note that fired cases from a rifle that had too-tight headspace would be on the short side after the headspace was adjusted, creating a headspace condition. If you are using the same brass this may be the problem, or at least part of it.
The way to test this would be to re-load a few of the brass that you have fired with the new bolt, using the same load. The shoulder should now be blown out to the new dimension, eliminating any headspace condition. If they behave OK, then you have identified the problem.
If this fixes it, then I'd toss all of the old brass and start off with a new batch. Firing all of the old brass to stretch it out could be dangerous. Remember that the nut behind the trigger is the only irreplaceable part.
Edited by charlesb (05/22/16 09:34 AM)
Kind regards, charlesb