Joe,
I'm not sure what is going on with my ability to post on your thread. I typed two nice long messages and both times it told me I didn't have permission to post and the message disappeared. I'll try the condensed version. I am puzzled over your question. It is so simple I never thought of it, yet I am having real trouble finding any kind of answer. I have looked through about 20-25 different documents dated from 1800 to 1979. That includes manuals, reports, books, complete round charts, etc. and NONE of them even mention the term semi-armor piercing associated with projectiles of any caliber. Around the turn of the century the classes were AP shot (solid projectile), AP Shell (cavity projectile) and Deck Piercing. In the 1940 printing of TM 4-205, Coast Artillery Ammunition ,it says that the old AP Shot and AP Shell projectiles were being phased out. Seacoast artillery projectiles 12-inches and above had the two older types being replaced by a more modern design that had an HE base cavity, an angular soft nose cap and over that a long tapered thin metal ogive and it was referred to as just an armor piercing projectile. A design very similar to many modern projectiles. Seacoast artillery projectiles less than 12-inches were being replaced by common steel high explosive shells, so obviously they felt no need for AP projectiles of any type in those calibers. Later manuals have APHE, AP Capped, AP shot, and later, of course, the HVAP family of projectiles. Nowhere in that chain of documents is the term SAP even mentioned that I found.
It is a little far fetched, but I wonder if the SAP could be the government's typical way of reversing words in nomenclatures and it was for Shot, AP or Shell, AP. Dosen't make much sense but other than a category of aircraft bombs I can find no reference to the term Semarmor piercing. I sure hope someone can clear this one up. Bob