The following will give an idea of the ballistic test as prescribed by the Bureau of Ordnance, Navy Department.  The test plate, irrespective of its thickness, is to be backed by thirty-six inches of oak or other substantial wood.  Near the middle region of the plate an equilateral triangle will be marked, each side of which will be three and one-half calibers long.  The lower side of the triangle will be horizontal.  Three shots will be fired, the points of impact being as near as possible the extremities of the triangle.  The velocity of the shot will be such as to give the projectile sufficient energy to just pass through a wrought iron plate of equal thickness to the test plate, and through its wood backing.  The velocity is calculated by the Gavre formula: 

a
V squared = —–­ { 3507 E squared x 2265464 e^{1.4} }
w

[TEX:  V^2 = \frac{a}{w} \{ 3507 \ E^2 \times 2265464 \ e^{1.4} \}]

V = the velocity of the projectile in feet per second. a = the diameter of the projectile in inches. w = the weight of the projectile in pounds.  E = the thickness of the backing in inches. e = the thickness of the plate in inches.

Using the above formula we can make out a table as follows: 

-------+-------+-------------+-------+-------+------+--

-------+
Plate. |Backi’g| Gun, service|  w,   |  a,   |  V.  | Energy, |
Inches.|Inches.| shot.       |Pounds.|Inches.| f. 8.| Impact. |
|       |             |       |       |      | f. tons.|
-------+-------+-------------+-------+-------+------+-------

--+
6    |  36   |   6” B.L.R. |  100  |  5.96 | 1389 |  1337   |
7    |  36   |   6”   "    |  100  |  5.96 | 1528 |  1619   |
8    |  36   |   8”   "    |  250  |  7.96 | 1213 |  2550   |
9    |  36   |   8”   "    |  250  |  7.96 | 1308 |  2966   |
10    |  36   |   8”   "    |  250  |  7.96 | 1399 |  3390   |
11    |  36   |   8”   "    |  250  |  7.96 | 1489 |  3839   |
12    |  36   |  10”   "    |  500  |  9.96 | 1247 |  5386   |
13    |  36   |  10”   "    |  500  |  9.96 | 1315 |  5987   |
14    |  36   |  10”   "    |  500  |  9.96 | 1381 |  6608   |
15    |  36   |  12”   "    |  850  | 11.96 | 1215 |  8699   |
16    |  36   |  12”   "    |  850  | 11.96 | 1269 |  9710   |
17    |  36   |  12”   "    |  850  | 11.96 | 1332 | 10454   |
18    |  36   |  12”   "    |  850  | 11.96 | 1374 | 11124   |
19    |  36   |  12”   "    |  850  | 11.96 | 1425 | 11965   |
20    |  36   |  12”   "    |  850  | 11.96 | 1476 | 12837   |
-------+-------+-------------+-------+-------+------+-------

--+

No projectile or fragment of the plate or projectile must get wholly through the plate and backing.  The plate must not break up or give such cracks as to expose the backing, previous to the third shot.

The penetration of projectiles of different forms into various styles of armor has been very thoroughly studied and many attempts have been made to bring the subject down to mathematical formulae.  These formulae are based on several suppositions, and agree very closely with results obtained in actual experiments, but there are so many varying conditions that it is extremely doubtful if any formulae will ever be written that will properly express the penetration.