United States of America
12"/35 (30.5 cm) Mark 1 and Mark 2
Updated 19 September 2008
In 1890 the Policy Board called for a variety of large caliber weapons, all the way up to 16" (40.3 cm) although this caliber was beyond the technology of the day.  A lighter-weight heavy weapon was also desired for long-range battleships and coastal monitors, which led to the development of the 12"/35 (30.5 cm).

The Mark 1 was fitted to the Second-Class Battleship Texas and some monitors, the very similar Mark 2 was fitted to the USS Iowa B-4.

In 1908 AP projectiles were fitted with a more streamlined ballistic cap of 7crh which improved their long-range penetration ability.

Background note:  The propellants used for most of the 19th century burned very quickly, giving the projectiles a short, sharp kick.  The longer the barrel the slower the muzzle velocity due to friction after the propellant had already finished combustion.  That is why heavy guns of this era had such short barrels, some only 20 calibers long.  The late 1800s saw the development of brown, or cocoa, powder which burned more slowly, but wasted much of its energy producing thick smoke.  This propellant allowed longer, higher muzzle velocity weapons to be developed.

The Mark 1 consisted of A tube, jacket, ten hoops and a locking ring, all constructed of gun steel.  The Mark 2 was similar, but I lack construction details.
WNUS_12-35_mk2_Iowa_pic.jpg

Battleship Iowa B-4 in 1898
Note the large gunports and the 8" (20.3 cm) secondary guns on the right
U.S. Naval Historical Center Photograph # NH 87
WNUS_12-35_mk1_Texas_pic.jpg

12"/35 (30.5 cm) gun on Second-class Battleship Texas
U.S. Naval Historical Center Photograph # NH 93700
WNUS_12-35_mk1_Texas_unloading_pic.jpg

Unloading projectiles on Texas
These appear to be both AP and Common projectiles
Detroit Publishing Company Collection Photograph
Library of Congress Photograph ID LC-D4-21010
WNUS_12-35_mk1_Texas_shells_pic.jpg

Unloading 12" (30.5 cm) shells from Texas
Detroit Publishing Company Collection Photograph
Library of Congress Photograph ID LC-D4-21011
.
Gun Characteristics
.
Designation 12"/35 (33 cm) Mark 1 and Mark 2
Ship Class Used On Mark 1:  Texas (1896), Puritan (M-1) and Monterey (M-6)
Mark 2:  Iowa (B-4)
Date Of Design N/A
Date In Service 1896
Gun Weight 102,550 lbs. (46,516 kg) (including breech)
100,800 lbs. (45,722 kg) (without breech)
Gun Length oa N/A
Bore Length 420 in (10.668 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist Increasing RH 0 to 1 in 25
Chamber Volume N/A
Rate Of Fire about 1 round per minute
.
Ammunition
.
Type Bag
Projectile Types and Weights AP - 870 lbs. (394.6 kg)
Common - 870 lbs. (394.6 kg)
Bursting Charge N/A
Projectile Length N/A
Propellant Charge 160 lbs. (72.6 kg)
Muzzle Velocity 2,100 fps (640 mps)
Working Pressure 15.5 tons/in2 (2,440 kg/cm2)
Approximate Barrel Life N/A
Ammunition stowage per gun Texas:  80 rounds
Iowa:  60 rounds
Others:  N/A
.
Range
.
Elevation With 870 lbs. (394.6 kg) AP Shell
15 degrees
(maximum elevation of turrets)		 About 12,000 yards (10,970 m)
30 degrees 21,000 yards (19,200 m)
.
Armor Penetration with 870 lbs. (394.6 kg) AP Shell
.
Range
Side Armor
Deck Armor
6,000 yards (5,490 m)
14.6" (371 mm)
---
9,000 yards (8,230 m)
11.6" (295 mm)
---
12,000 yards (10,920 m)
9.4" (239 mm)
---
Data is for face-hardened Harvey plates from "Ordnance Data Sheets" of 1905 and is for a muzzle velocity of 2,600 fps (640 mps).
.
Range
Side Armor
Deck Armor
6,000 yards (5,490 m)
12.9" (328 mm)
---
9,000 yards (8,230 m)
10.7" (272 mm)
---
12,000 yards (10,920 m)
8.7" (221 mm)
---
This data is from "Elements of US Naval Guns" of 1918 and is for the 7crh projectile at a muzzle velocity of 2,400 fps (732 mps).  Data is corrected for angle of fall and may also refer to harder armor than used for the 1905 data. 
.
Mount / Turret Data
.
Designation Single Turrets
   Texas (2):  Mark 2

Twin Turrets
   Puritan (2) and Monterey (1):  Mark 1
   Iowa (2):  Mark 3
Weight Mark 1:  N/A
Mark 2:  N/A
Mark 3:  448 tons (455 mt)
Elevation Mark 1:  -3 / +15 degrees
Mark 2:  -5 / +15 degrees
Mark 3:  -3 / +14 degrees
Rate of Elevation N/A
Train about -150 / +150 degrees
Rate of Train N/A
Gun Recoil N/A
Loading Angle Mark 1:  +9.5 degrees
Mark 2:  0 degrees
Mark 3:  +3 degrees
Notes:

1) The Mark 2 turrets on Texas originally could only load at 0 degrees train angle as well as at 0 degrees elevation.  This because the hydraulic rammers for these guns were located outside of the turrets in the fixed structure and so did not rotate with the turrets.  Shortly before the start of the Spanish-American War, internal telescoping rammers were fitted inside of the turrets which then allowed the guns to be loaded at any angle of train.

2) The Mark 3 was the first balanced rotating turret in USN service.  It could be loaded either by hand or with an electric rammer.

3) The monitor Monterey was unusual in that she had a twin 12"/35 (30.5 cm) turret forward and a twin 10"/30 (25.4 cm) turret aft.
.
Data from
"US Naval Weapons" and "US Battleships:  An Illustrated Design History" both by Norman Friedman
"Battleships" by Paul Stillwell
and
Tony DiGiulian's personal files