PROBLEMS INVOLVED IN EVALUATING ARMOR PENETRATION
By NATHAN OKUN (8/29/2018)

When trying to evaluate ballistic test results for armor penetration, there are a large number of things that cause basic problems.  To wit:

	(1)  Lack of enough information -- required data on velocity, angle of impact, plate thickness, plate properties allowing it to be classified as to what formulae and constants apply, projectile weight, projectile type (including AP capped or uncapped, hard or tough or soft AP cap attached), and so forth.  This means either throwing out the results or guessing the needed information from other context, neither one is really acceptable for firm analysis.  Sometimes this guessing can be done with a high probability of success, but other times it is really shaky and can only be trusted to give some rough ideas as to what to expect when other conditions apply.

	(2)  Lack of a known definition of "Ballistic Limit" when the database calls out a projectile's "Success" or "Defeat" by the plate.  For example, does it mean that the projectile remains "Whole" or "Intact" or "Entire" or "Fit to burst" or "Effective" or "Excellent" (close to my Effective Ballistic Limit or EBL) or just that it punches completely through the plate in any condition, even in small pieces (a high percentage of it, at least), as the standard US Navy Ballistic Limit (NBL) requires?  Not knowing this can cause really biased results if the wrong limit is chosen for an evaluation.  Very frustrating at times!

	(3)  Conflicting test results.  This means that one test gives results that totally contradict another result or results, making it unclear as to what the true meaning of the test series is.  All projectiles lose their AP caps and windscreens on impact.  Term "head" is projectile tapered nose under the AP cap.  The following table for test of the British post-Jutland "Greenboy" 12" Mk VIIA APC projectiles (Full Weight 854 lb, 747.4 lb w/o AP cap or windscreen) made by Vickers at the end of WWI gives such a mess:

VICKERS 12" Mk VIIA APC SHELLS VS THREE NEW KC PLATES @ 20° OBLIQUITY
(From APPENDIX "F" to Unknown Document in British National Archives File ADM186/169)
  PLATE	| TEST #	| CAP?	| TEST V	|     PLATE DAMAGE	|         PROJECTILE FINAL CONDITION
--------------------------------------------------------------------------------------------------------------------------------------------
Vickers 6"	|      1	|   Yes	|   960f/s	|     Hole 12.4" x 12"	|  Holing Only.  Entire (= no significant damage).
    ditto	|      2	|   Yes	|   774f/s	|     Pit 2.3" deep	|  Whole (= cavity intact) w/head snapped off.

    ditto 	|      3	|   No	|   970f/s	|     Pit 6.8" deep	|  Whole. 
    ditto	|      4	|   No	|   928f/s	|     Hole 15.5" x 13.5"	|  Holing Only.  Whole.
    ditto	|      5	|   No	|   952f/s	|     Pit 5.3" deep	|  Whole.
    ditto	|      6	|   No	|  1028f/s	|     Hole 12" x 11"	|  Holing Only.  Entire with no deformation.
    ditto	|      7	|   No	|  1199f/s	|     Hole (size unknown)	|  Complete Penetration.  Projectile #6 reshoot (!).
					      Broke up (= in several or many pieces).
    ditto	|      8	|   No	|  1069f/s	|     Hole 11.12" x 12.3"	|  Holing only.  Whole.

Vickers 8"	|      1	|   Yes	|  1155f/s	|     Pit 1.3" deep	|  Broke Up.
    ditto	|      2	|   Yes	|  1242f/s	|     Pit 2.1" deep	|  Broke Up.
    ditto	|      3	|   Yes	|  1380f/s	|     Hole 19.3" x 14.9"	|  Partial Penetration.  Broke Up. 

    ditto	|      4	|   No	|  1460f/s	|     Hole 15.4" x 17.6"	|  Partial Penetration.  Broke Up.
    ditto	|      5	|   No	|  1352f/s	|     Hole 15.5" x 14.1"	|  Holing Only.  Broke Up.
    ditto	|      6	|   No	|  1206f/s	|     Pit 2.25" deep	|  Broke Up.
    ditto	|      7	|   No	|  1267f/s	|     Hole 14.3" x 13.2"	|  Complete Penetration.  Whole. (!!!!)

Brown  8"	|      1	|   Yes	|  1258f/s	|     Pit 2" deep	|  Broke Up.
    ditto	|      2	|   Yes	|  1346f/s	|     Hole 18.6" x 14.6"	|  Complete Penetration.  Broke Up. 

    ditto	|      3	|   No	|  1258f/s	|     Pit 2.6" deep	|  Broke Up.     
    ditto	|      4	|   No	|  1326f/s |     Pit 4.4" deep	|  Broke Up.
    ditto	|      5	|   No	|  1344f/s	|     Hole 11.5" x 14.1"	|  Partial Penetration.  Broke up.  
These tests give rather odd results:

	(1)  The uncapped shell's shatter velocity at 20° is about 1100 f/s minimum.  It might be higher if the 6" (0.5-caliber plate) Shell Number #7 shell broke up due to stress breakup rather than true shockwave-induced shatter.  These shells were so tough against 6" plates at under 1100 f/s that Shell Number #6 could have a new driving band put on it and fired out the same gun again, meaning that the shell had no noticeable distortion due to the impact at over 1000 f/s that bounced backward (usually the worse kind of stress on such a shell).  This is well-nigh unbelievable! 

	(2)  Partial Penetrations are in-between the Holing Limit ("Holing Only" above) and the Complete Penetration Limit (NBL).  Part of the projectile's pieces were found behind the armor, but not enough to be considered a Complete Penetration   At the time of these tests, if any significant number of pieces of the projectile were found behind the plate, it was a "Perforation", not a "Penetration" (Holing Only or Partial Penetration), but by WWII, they seem to have changed the former to mean something similar to the US Navy NBL.  There are several impacts that did not completely or even partially penetrate at higher velocities than those that did.  Indicates a large Zone of Mixed Results.

	(3)  The only shell that completely penetrated an 8" plate whole was an UNCAPPED projectile, while all other shells broke up, no matter what they did to the plate.  This is totally backwards and to my knowledge, unique for this projectile in all tests that I know of.  Seeing that these new-model APC shells had a new hardened AP cap, one would have thought that the shell would give better performance than what is observed here.  Also note that this shell made the smallest size hole in the armor of any shell that partially or completely penetrated.  The smaller the hole, the less armor resisting the shell, so this plus the fact that the uncapped shell had minor damage after the impact may indicate that the shell hit a weak spot in the plate.

	(4)  This projectile essentially -- other than a rare fluke -- cannot penetrate an 8" (0.67-caliber) KC plate at 20°, the projectile maximum plate thickness for intact penetration (at most typical striking velocities in a battle) being somewhere between 6" and 8".  (NOTE: In a later comparative test series against the Russian 12" Model 1908 APC shell (with a so-called "tough" cap), these British 12" Mk VIIA APC results were confirmed completely.  The Russian shell was superior and could completely penetrate British 8" KC whole.)  This result is the worst of all of the Greenboy shells, these being the 9.2" developed later (for Coast Defense guns) and the 13.5" (both Light and Heavy), 14" (HMS CANADA), and 15" developed at the same time as the 12".  The 12" Mk VIIA APC shell was the lightest for its size of all of these improved APC shells and, to my knowledge, the only one that was completely of cast steel; all of the others were forged in presses and/or hammers to their final shapes from cylindrical steel ingots.  These two factors may account for the poor performance.  The other shells could easily penetrate 0.67-caliber British KC plate at 20° -- indeed, when used for testing various armor configurations for the then-new HMS HOOD, the 15" shell easily penetrate 7" (0.47-caliber) British KC armor at 40° (!!!) in intact condition and US tests of the British 14" APC shell penetrated a 13.5" (0.96-caliber) US Carnegie KC Class "A" (face hardened) plate at 30° in Whole (= US Navy "Effective") condition, which at the time was the best result for any APC shell that I know of.

	(5)  The Vickers 8" KC plate was significantly superior to the Brown plate, since the Vickers plate's NBL was above 1460 f/s, while the Brown plate could have its NBL for this shell at 20° set to 1345 f/s with no conflicting results.  This is a typical plate-to-plate spread for KC armor (some armors are even worse).  I would need more tests than just these plates to get a good handle on this projectile's quality and the average plate quality for British KC at this time.  Luckily I did have more and used them in my FACEHARD program. 

As can be seen, there are several conflicting results in this test series when testing 8" armor, though 6" armor has almost no effect on the projectile at all (at 20°, at least).  These shells may also have been able to penetrate 6" plate whole at 30 degrees (no data here, though), which was the German Krupp C/11 (1911 Model) APC shell requirement -- this puts these new British 12" APC shells somewhat "behind the curve" as to current shell quality in foreign navies at the end of WWI, though the new Greenboy shells for other British large-caliber guns were significantly better.  It might be possible for this shell to penetrate 8" KC plate intact at a higher velocity than any used here, but there is no data on what this would be from these tests, so I would have to guess at something.