BRITISH WWI-ERA APC SHELL AP CAP & WINDSCREEN WEIGHTS
Transcribed by Nathan Okun (8/25/2018)

This table is based on two official British documents:  One a page giving all British APC shell combined AP Cap and Windscreen weights used during WWI (both pre-Jutland and "Greenboy" post-Jutland -- painted green to make sure that they were not mixed up with the older APC shells!) in pounds and as percentage of the total projectile weight (part of British National Archives section ADM186/169, document title unknown) and the other being British C.B. 01303, "Final Report of the President, Projectile Committee, 1917".

After WWI, the thin, sheet-steel, streamlined Windscreen to reduce aerodynamic drag and increase range of the shell was given its technical name, "Ballistic Cap" (BC), used by the British Army and Navy and the US Army.  The US Navy, however, continued to use the terms "Windscreen" or "Windshield" when talking about these nose coverings.  They were employed starting in the first decade of the 20th Century when longer-range gunfire became possible due to improved range-finding and fire-control equipment.  The original British term was "Deflector" and referred to the rather tiny (0.5-1.5% of projectile total weight) truncated cone, hemispherical, or elliptical Windscreens covering just the tip of the new tapered (conical or ogival) AP Caps then being introduced.  These AP Caps, due to rather strange ideas as to "Cap Action" in protecting the projectile nose under them, had, if possible, even worse streamlining than the oldest AP Caps, which did at least have a flattened cone as a face to reduce drag a little.  Most of the new AP Caps absolutely needed to be hidden during flight.

Note that, while some streamlined AP Caps were shaped to look like the ones with deflectors and thus did not use a Windscreen -- for example, early French AP Caps and all British Cammel Laird AP Caps (see below) -- all other "Armor Piercing" (AP) shells with poorly-shaped AP Caps did require them during and after WWI.  In many cases, however, the Windscreen was simply assumed to exist and the shell ID descriptor "BC" or its equivalent in another language was dropped.  For example, WWI and after British Navy "APC" shells almost always had a deflector or, later, a large Windscreen, but the "BC" was not used -- this even eventually happened to US Army ammunition during WWII also -- even though a deflector, as mentioned, usually existed in its improved-drag nose designs, while the US Navy went even farther and simply assumed both an AP Cap and a Windscreen was always present in its "AP" shells from a time just before WWI (if the shell had no AP Cap, it was a base-fuzed "Common" projectile, no matter how heavily constructed to get maximum armor penetration), though a few US Navy AP shells for its older guns still used tapered AP Caps and had no Windscreens (these were mostly replaced by new projectiles during WWI, anyway).  Thus, the existence of a Windscreen on a given AP shell is in many cases not obvious unless you already know about the idiosyncrasies of the projectile designs in use by a given nation at a given time.

The "A" after the Mark # specifies a standardized secant ogive ballistic streamlined nose shape with a small hemispherical tip, formed either by the AP Cap tipped with a rather small Windscreen as used with the pre-Jutland APC shells or, in some APC shells, by having solid AP Caps of that shape.  The Greenboy APC shells had blunter projectile noses under their caps and thicker, wider, very blunt AP Caps, so their Windscreens were much longer and weighed much more to make that shape.  To my knowledge, all Greenboy and 15" Mk VA APC shell caps were either the Hadfield very-flat-cone-faced or the "Knob-and-Ring" (central dome imbedded in a wide flat disk-shaped face), both of which were hardened, the first British hardened AP Caps.  Later, post-1930 AP Cap designs replaced the Greenboy Hadfield cap with a rounder, contoured Hadfield AP Cap design that had more uniform thickness over the entire nose for improved oblique impact performance, though the Knob-and-Ring design was also widely used.  The much more streamlined type "B" ballistic nose shape was introduced in the late 1920s, but "A" shells were still used in WWII.

In the table on the next page, the British 12" APC designs show the progression in AP Cap and Windscreen development in the British Navy from about the year 1900.  The 12" Mark VI -- and its immediate predecessor the Mark IV (the latter still widely used in armor-piercing testing until well after WWI, when the final Mark VIIA design replaced it through the end of WWII for armor testing requiring a 12" gun) -- was basically an uncapped AP projectile with a rather blunt-faced AP Cap "jury-rigged" on using crimping into a ring of ground out pits in the lower nose to hold it on (later British and US Navy AP caps still used this crimping technique even though the much stronger soldering attachment technique became the primary method of holding on the cap after WWI in every country's new naval ammunition).  Most original AP Caps were variants of the US-designed "Johnson Cap" which looked rather like a pill-box stuck on the projectile nose as an after-thought (rather high drag), which it really was when AP Caps were finally recognized as required against face-hardened battleship side armor around the year 1900 or so by all nations with navies and many older AP shells were retro-fitted with them "en masse".  When low-drag, standardized ballistic shell designs were introduced just before WWI (1911 in the US Navy, for example), many older shells had their AP caps replaced with lower-drag shapes with or without deflectors; here, the British 12" Mark VIA APC shell redesign.  Finally, WWI battle results indicated to the British that they needed a new APC shell with a stronger body and a hardened, thicker AP Cap, so they designed the post-Jutland "Greenboy" 12" Mark VIIA APC projectile, with a blunter projectile nose under the AP Cap and a flatter, wider face on its new hardened AP Cap type, which needed a much larger Windscreen to regain its type "A" drag status.  Note that the AP Cap weight in the case of the 15" Greenboy projectiles did not change much, even though the Windscreen became much heavier, since a hardened AP Cap was found to need to be wider to handle oblique impact but did not to need to be quite so thick to do its job, when tested after Jutland, even with the new Greenboy 20-degrees-obliquity against roughly half-caliber KC side armor intact penetration requirement added.  Note that the 12" Mark VIIA APC shell was both the lightest in weight for its size AND was the worst of the new Greenboy APC shells in remaining intact ("Fit to burst" or "Whole") after penetration, not being reliable against 8" (0.67-caliber) KC armor at 20 degrees obliquity, while the larger sizes of these new shells could usually penetrate scaled-up 0.67-caliber KC armor even at 30 degrees with no problem.  Indeed, one US test of a new-type British 14" APC shell had it completely penetrate a KC plate of 13.5" thickness at 30 degrees with minimal damage, far exceeding any other shell known at the time at such a then-very-highly-oblique angle -- though this later became the standard test angle in most nations' WII APC shell requirements specifications.

PROJ DIAM  	|      15"	|   14"**	| 13.5"(Heavy)	| 13.5"(Light)	|           12"              
PROJ MARK # 	| IA     IIIA*	| IA   IIIA*	| IA     IIIA*	| IIA     IVA*	| VI     VIA****     VIIA*
PROJ WEIGHT 	|1920    1910	|1586  1595	|1400    1410	|1286.5   1257	|850      859.7       854
___________________________________________________________________________________________________

MANUFACTURER

HADFIELD
COMBINED WT	|145     213.5	|118.5  N/A	| 78.8     148	| 49     147  	| 73       34.8      98
% CAP/WS***	|6.6/1 8.7/2.5	|6.5/1  	|4.6/1   8/2.5	|3.8/0  9.2/2.5	|8.6/0      4/0       9/3

FIRTH
COMBINED WT	|158.5   201.8	| N/A   N/A	| 75.6   155.5	| 52.7   152.7	| 78.8     30        N/A
% CAP/WS***	|7.3/1 8.1/2.5	|    	|4.4/1 8.5/2.5	|3.1/1  9.6/2.5	|9.3/0    2.5/1       
 
VICKERS
COMBINED WT	|158.5   221.8	| N/A   N/A	| 88.5   153.8	| 42.3   155	| 78       29.7     106.7
% CAP/WS***	|7.3/1 9.1/2.5	| 	|5.3/1 8.4/2.5	|2.3/1  9.8/2.5	|9.2/0    2.4/1     9.5/3

E.O.C.
COMBINED WT	|159     233.5	|127   177.5	| 77     167.8	| 46.7   167	| N/A      30       107.7
% CAP/WS***	|7.3/1 9.7/2.5	|8/1 8.6/2.5	|4.5/1 9.4/2.5	|2.6/1 10.8/2.5	|         2.5/1     9.6/3

CAMMEL LAIRD
COMBINED WT	| N/A    228.5	| N/A   N/A	| 88.5   171.5	| 41.8   150   	| 81.5     35        98.7
% CAP/WS***	|         12/0	|	|6.3/0  12.1/0	|3.2/0 11.9/0  	|9.6/0    4.1/0    11.6/0

NOTES:
*Greenboy Post-Jutland Shell (the 1920s 15" Mark VA APC shell was an improved version of this).	
**HMS CANADA only.  Later 14" Mark IB and VIIIB APC projectiles were totally new post-1930 designs.
***Windscreen percentages use C.B. 01303 and other projectile pictures/diagrams (such as similar WWI-era US Navy AP shells) to estimate it and thus also the AP Cap percentage.  Windscreen percent estimate is to the nearest 0.5% of total projectile weight, so this is also true of AP Cap.  Lighter shells of the same Windscreen shape and size (i.e., 12" Mark VIIA APC) had proportionately heavier Windscreens.  The shell's total length was limited by the handling/storage system in the oldest ships using it.
****Note that this shell had a totally redesigned, lighter-weight AP Cap and, finally, a Windscreen, compared to the previous, very blunt, heavy-AP-Cap-only Mark VI APC high-drag nose (this varied with manufacturer).  The thinner, tapered AP Cap demonstrated that the AP Cap thickness was not very important when soft caps were employed, at least at very low obliquity angles well under 20 degrees (these older, pre-Jutland APC shells had only to pass a normal-impact intact-penetration test standard).  Other than the new AP Cap and Windscreen, I do not know what changes were made to the Mark VI to turn it into the Mark VIA.  The Mark VIIA had a smaller internal cavity (2.5% instead of the Mark VI/VIA 3.3%) making the shell heavier for its size, so the lower final weight -- compared to the Mark VIA with the same nose contour -- of that large AP Cap and Windscreen meaning the shell body itself was shortened significantly, with a blunter nose under the cap -- about a 1.5-caliber-radius tangent ogive (arc of circle) shape compared to the older shell's more pointed 2-caliber radius -- to resist damage at an oblique impact.