You may be onto something, when Paladin chambered the blank he used the "standard" of ten thou beyond exact headspace. I did not have him check to see how the chamber in my DE compares to that before cutting it though. I also wonder if it may have anything to do with the bolt unlocking in the AR. The pistol length gas tube on an 11.5" barrel even with only an .080" gas port had plenty of gas. Before I started shooting it suppressed I closed off my adjustable gas block by more than half and it was still cycling enough unsuppressed to lock the bolt back on the face of the carrier. I may close the gas off completely and see if there is any difference in how the primer looks.I wonder if this has anything to do with headspacing.
The long 50AE brass you speak of from Starline is how the first runs of 458 Socom brass were manufactured. I wonder if they would be willing to accommodate that request, hard to say though.
Those loud wonderings can sometimes yield the best answers!