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So I finally got my hands on some N140 and made a preliminary model for my 20" gas gun. This is just a blank slate that I filled with my numbers, as of now un-tested and not confirmed.
1) I simply replaced the N140 for W748 and this is what it produced.
2) W748 is not well represented in GRT powder, meaning there are not a lot of submissions to "fine tune" the powder character.
Caveat lector
SI VIS PACEM, PARA BELLUM