Happy to participate, Danny. : )
I fear, though, that you've asked a question on a subject which few of even bullet-collectors have paid deep-enough (no pun intended) attention to. "Enfield Cone-cavities deeper than .4-inch." For example, I myself have bothered to actually MEASURE a con-cavity's depth only very recently. I suspect that the great majority of folks just think "hey, that's an uncommonly deep cone" and let it go at that.
Anyway, doing some checking at your behest, I do have a few cone-cavity Enfields that are deeper than .4-inch. I think their "few-ness" in my collection is significant, due to the fact that ever since I got my start in relic-hunting in the Atlanta Campaign area (where Enfields are by far the most common CS bullet), I've been noticing and collecting CS-CAST Enfield variants. Thus, I've accumulated a good number of some really odd CS-Enfield cavities ...but few that are deeper-than-.4-inch cones.
I should mention that (at least among my specimens) they tend to have an unusually thin "skirt."
For whatever it's worth, I can testify that they are found in "noticeable" - but not large - numbers in the Atlanta Campaign trenches. (From Dalton to Atlanta, not just around the city itself.) I should also mention that a large percentage of the Enfields we "North-Georgia" diggers found had a plug-cavity (meaning, British-made) - which seems a bit surprising for a mid-1864 Western-Theater campaign. I can only suppose they were coming in from the port of Wilmington NC (one of the few blockade-runner ports still open at that point of the war - Savannah and Charleston, though uncaptured, were quite effectively bottled-up). Also, perhaps not coincidentally in regard to Wilmington imports, there were a lot of North Carolina regiments in Joe Johnston's Army of Tennessee in 1864.
I suspect that if the Atalanta Campaign plug-cavity Enfields were coming in from the port of Mobile, we'd have found significantly more Selma Arsenal minies also. But the Selmas were few and far between.
I've also lived and dug extensively in the Richmond area and the Fredericksburg/Spotsylvania area. In those places, the Enfield minies you're interested in are found far less often than in the Atlanta Campaign ...probably due to Richmond's situation as the greatly-predominant producer of Gardner minies.
Danny, I hope this info is at least somewhat helpful.
Regards,
Pete [PCGeorge]